Galaxies are very massive objects, made of billions of stars, of course. They attract each other to form galaxy clusters and superclusters. But could there be an object with the mass so huge, entire galaxy clusters circle around it?
At the centre of Laniakea supercluster, there’s a gravity anomaly called the Great Attractor which seems to pull all surrounding galaxies toward it. It is estimated that its mass is tens of thousands times that of the Milky Way. To put it in the perspective, the most massive galaxy we know has “only” 20-35 times more mass than our galaxy. Laniakea supercluster is actually our home, since the Milky Way is the part of it.
What would create such an anomaly is not known at the time. The Great Attractor is in the part of the sky that is obscured by the dust in the Milky Way, so we have difficulties observing it. It has been discovered in 1986 after a deviation from uniform expansion of space was reported in 1970s i.e. galaxies in that area did not move the way they were supposed to. This gave astronomers a clue that there must be a very massive object attracting galaxies toward it.
We know that the Great Attractor lies some 250 million light-years from us between constellations Triangulum Australe and Norma, and we know that at its centre, there is Norma cluster some 1000 times heavier than the Milky Way.
Could it be that the Great Attractor is just a supercluster we cannot see? Sure, it is always possible, but we can observe Norma cluster with X-ray telescopes and it lies near the Great Attractor. So why can’t we see it with our X-ray telescopes?
While its name gives an impression that everything is pulled toward it, some recent studies have indicated that the entire Laniakea supercluster, together with the Great Attractor, is pulled toward even bigger supercluster called Shapley supercluster, located beyond the Great Attractor, some 650 light-years away. If you ever wondered why it took so long for us to realise that Earth was orbiting the Sun and not unmovable centre of the Universe, keep in mind that humans then probably felt like we do today, unable to observe the movement of what is essentially our home.