There are galaxies so large that comparing them with the Milky Way almost stops making sense.
IC 1101 is one of them.
For years, astronomers have known that this enormous galaxy sits at the heart of the Abell 2029 galaxy cluster, roughly a billion light-years away.
It has often been described as one of the largest galaxies known.
But there was a problem.
Nobody could confidently say where the galaxy actually ended.
Its outer regions are extraordinarily faint, gradually blending into the surrounding light of the galaxy cluster.
Now, new ultra-deep observations have pushed that boundary much farther out.
And the result is extraordinary.
IC 1101 appears to stretch for approximately 1.7 million light-years. That is around 17 times the diameter of the Milky Way's main stellar disk.
And the really interesting part?
It may still be growing.
A galaxy at the centre of everything
IC 1101 isn't an ordinary galaxy drifting through space. It is a brightest cluster galaxy, or BCG.
It sits near the gravitational centre of Abell 2029, an enormous cluster containing thousands of galaxies.
Galaxy clusters are enormous gravitational structures. Galaxies orbit within them, interact with one another and occasionally collide.
Over billions of years, a galaxy positioned at the centre of such a system can accumulate enormous quantities of stars.
IC 1101 appears to be the ultimate example.
Rather than simply forming stars throughout its history like a spiral galaxy such as the Milky Way, much of its extraordinary size is thought to have developed through the accretion and destruction of other galaxies.
In other words:
IC 1101 grew by eating its neighbours.
How big is 1.7 million light-years?
Numbers this large are almost impossible to visualise.
The Milky Way's main stellar disk is roughly 100,000 light-years across. IC 1101's newly measured stellar extent is approximately 1.7 million light-years.
If the two galaxies could somehow be placed side by side, the Milky Way would look remarkably small.
But there is an important caveat.
Galaxy sizes aren't as straightforward as measuring the diameter of a planet. A planet has a relatively well-defined surface. A galaxy doesn't.
Stars become progressively more sparse as you move outward.
Eventually the light becomes so faint that distinguishing the galaxy from the surrounding background becomes extremely difficult.
So when astronomers say a galaxy is “X light-years across”, they are really defining an observable boundary based on the distribution and brightness of its stars.
This is precisely why IC 1101 has been so difficult to measure.
The missing edge
For years, estimates of IC 1101's size varied dramatically. Some measurements produced figures of several hundred thousand light-years. Others suggested an even larger stellar envelope.
The problem wasn't necessarily that astronomers disagreed about the galaxy's existence or structure. It was that its outer regions were simply too faint.
In 2026, a team led by Carlos Marrero-de la Rosa used exceptionally deep images from the Isaac Newton Telescope's Wide Field Camera to investigate those outer regions.
The researchers found a coherent stellar envelope extending approximately 260 kiloparsecs from the galaxy's centre.
That corresponds to a diameter of about 520 kiloparsecs, or roughly 1.7 million light-years.
For the first time, astronomers were able to put a much stronger observational boundary around the giant.
And the result placed IC 1101 at the extreme upper end of known galaxy sizes.
The stars alone are astonishing
The new study estimates approximately 3.4 trillion solar masses of stars within the measured stellar extent.
That's an extraordinary number. For comparison, NASA estimates the total mass of the Milky Way, including its dark-matter halo, at around 1.5 trillion solar masses.
This means something rather remarkable.
The estimated mass of the stars in IC 1101 alone exceeds the estimated total mass of the entire Milky Way.
But comparisons need to be handled carefully.
The measurements use different definitions and uncertainties, and a galaxy's total mass includes dark matter as well as stars.
Nevertheless, the scale difference is enormous. IC 1101 is not simply a slightly larger version of our galaxy.
It belongs to another category of galactic giant.
How does something become this enormous?
The answer appears to involve one of the most violent processes in the universe: galaxy mergers.
Galaxies are not solid objects. When two galaxies collide, individual stars almost never physically smash into one another. The distances between stars are simply too great.
Instead, their gravitational fields interact.
Gas clouds can collide. Orbits can be disrupted. Star formation can be triggered. And stars from one galaxy can become gravitationally bound to the other.
Over time, two galaxies can become one. Repeat the process enough times and an enormous central galaxy can emerge.
This is one of the reasons brightest cluster galaxies can become so huge.
They occupy the gravitational heart of their clusters, where smaller galaxies are continually passing through the surrounding environment.
Some eventually become part of the central giant.
IC 1101 may be a galactic graveyard
This creates a fascinating way to think about IC 1101. It may not simply be a galaxy. It may be a record of billions of years of galactic destruction and assembly.
Every merger can leave clues. Stars from different galaxies can have different chemical compositions and ages. Their orbits can carry information about where they came from. Faint streams and asymmetrical structures can preserve traces of ancient gravitational encounters.
The outer regions of IC 1101 are therefore particularly valuable. They may contain some of the clearest evidence of the galaxy's violent past.
And the latest observations have revealed several faint, asymmetric structures around its outskirts. The researchers interpret these features as evidence consistent with ongoing mass assembly.
The monster may not be finished growing.
A giant with a strangely empty centre
IC 1101 has another unusual feature. Its central region contains an extraordinarily large depleted core. Astronomers studying the galaxy found a core roughly 4.2 kiloparsecs in radius, among the largest measured in any galaxy.
That is significant because depleted galactic cores can be produced when supermassive black holes interact during galaxy mergers.
When two galaxies merge, their central black holes can eventually form a binary system. As the black holes interact gravitationally, they can scatter surrounding stars outward.
The result can be a region near the centre with fewer stars than expected.
This process is sometimes described as “scouring” the galactic core.
A 2017 study of IC 1101 found evidence consistent with such a process and estimated a very large stellar mass deficit in its core.
It provides another tantalising clue that IC 1101 has experienced an extraordinary history of mergers.
And then there's the black hole
At the heart of a galaxy this enormous, you would expect a supermassive black hole.
IC 1101 appears to have one.
But here the numbers become considerably less certain. Various indirect methods have produced extremely large estimates for its central black hole, potentially tens of billions of solar masses.
Some modelling of Abell 2029 has used a black-hole mass of approximately 50 billion solar masses.
However, this should not be presented as a precisely measured number.
Direct measurements of such distant black holes are extraordinarily difficult. The enormous estimates remain model-dependent.
That uncertainty is actually interesting. Because if the black hole really is anywhere near this massive, IC 1101 would possess one of the most extreme black holes known.
The galaxy that ate galaxies
There is a temptation to imagine IC 1101 as some kind of cosmic monster roaming through space and consuming everything it encounters.
Reality is slower.
There is no galactic predator chasing individual galaxies. Instead, gravity operates over immense periods of time.
A smaller galaxy enters the cluster. Its orbit changes. Tidal forces distort it. Stars are stripped away. The smaller galaxy may eventually merge with the central giant. Then another arrives. And another.
Over billions of years, the accumulation becomes enormous.
The process is less like an animal hunting prey and more like a cosmic gravitational snowball. But the end result can look just as spectacular.
Is IC 1101 still growing?
This is perhaps the most exciting part of the latest research. The new observations identified faint structures around IC 1101 that appear to be associated with ongoing disturbances in the surrounding cluster environment.
The researchers conclude that its outskirts show signs of ongoing mass assembly.
That means the largest known galaxy may not have reached its final size. It could still be accumulating stars. It could still be absorbing smaller systems. Its outer envelope could continue expanding.
The galaxy we observe today may therefore be only one stage in its evolution.
But will it grow forever?
Probably not. The universe is expanding. Galaxies inside gravitationally bound structures can continue interacting, but the expansion of space increasingly separates distant, unbound structures.
Eventually, the supply of nearby galaxies available for major mergers will diminish.
Within a bound galaxy cluster, however, gravitational interactions can continue for extraordinarily long periods.
The future of IC 1101 is therefore likely to involve further mergers and gradual evolution rather than endless growth.
Eventually, there simply may not be many large galaxies left nearby for it to absorb.
What would it look like from inside?
Imagine, for a moment, that we could somehow transport Earth to a planetary system inside IC 1101.
The experience wouldn't necessarily be like living inside a giant version of the Milky Way.
IC 1101 is an elliptical galaxy. It lacks the prominent spiral arms and extensive thin disk that define our home galaxy. Star formation is also much less active than in galaxies rich in cold gas. The sky could nevertheless be spectacular.
Depending on our location, enormous numbers of stars would fill the darkness. And somewhere far away would be the distant outer edge of a galaxy extending for more than a million light-years.
But there would be no obvious boundary.Just stars becoming progressively more sparse, until eventually, the galaxy faded into the surrounding universe.
The biggest galaxy — but with a warning
There is one phrase that deserves caution: “The biggest galaxy in the universe.” We cannot honestly make that claim.
The observable universe contains an enormous number of galaxies, and measuring the physical boundaries of the largest systems is extremely difficult.
There may be galaxies we haven't measured deeply enough. There are also different ways of defining “size”.
The latest study makes a strong case that IC 1101 is the largest galaxy measured so far by this particular stellar-extent definition, with an approximately 520-kiloparsec diameter.
That is impressive enough without exaggeration.
A galaxy built from other galaxies
Perhaps the most fascinating thing about IC 1101 isn't its size. It is what that size represents. The galaxy may be a visible consequence of billions of years of cosmic assembly.
Ancient stars were incorporated into an ever-growing stellar envelope. And slowly, almost imperceptibly, a giant emerged.
Today, light from IC 1101 travels across roughly a billion years of cosmic history before reaching our telescopes.
We aren't seeing the galaxy as it exists today. We are seeing it as it was when that light began its journey. And even then, the latest observations suggest that the story was still being written.
The monster at the centre of the cluster
IC 1101 gives us an extraordinary glimpse of what gravity can accomplish when given enough time.
Our Milky Way is enormous by human standards. But beside IC 1101, it is a relative dwarf.
And perhaps that is the real lesson. The universe doesn't merely contain big things. It contains objects whose scale challenges the very way we think about size.
A galaxy can become so enormous that its outer boundary is almost impossible to find. It can contain trillions of solar masses in stars. And it can preserve evidence of countless mergers. It can continue gathering material from its surroundings billions of years after its formation.
IC 1101 isn't a static monument. It appears to be a galaxy still under construction.
And somewhere, more than a billion light-years away, gravity may still be building one of the largest structures a single galaxy can become.
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