Eli5: What is the great attrctor and laniakea?
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The universe is expanding.
Take two points in space, and a second later they will be ever so slightly further apart than they were.
This is a really tiny effect (think a 0.00…01 with 18 0s in there). It is meaningless on “small” scales like within galaxies, and only really matters between clusters of galaxies.
Given this, clusters of galaxies should appear to be moving away from each other, in a neat, predictable way.
But they don’t seem to be. The Milky Way (our galaxy) and a bunch of galaxies (and clusters of galaxies) around it seem to be moving in a particular direction. The “Great Attractor” is the proposed explanation for this; that there is some mass (or combination of masses) in a particular region of space, whose gravity is strong enough to be pulling our local galaxy clusters towards it, when they should be moving apart. It would need to be a concentration of mass millions of times more massive than the Milky Way.
Unfortunately the point in space where it should be is the other side of the Milky Way from us, making it very difficult to see.
The Laniakea Supercluster is the name given to the 100,000 or so nearby galaxies that seem to be drawn into this point. They are our local neighbours. It is defined kind of like the “drainage basin” of the Great Attractor; stuff that appears to be drawn into this region of space is part of the Laniakea Supercluster, stuff that isn’t is on the outside.
This [neat little animation](https://en.wikipedia.org/wiki/File:Nearby_Superclusters.webm) shows some of our local superclusters – the ones marked Laniakea are the ones in the Laniakea Supercluster, so appear to be drawn towards this Great Attractor thingamy. The others aren’t.
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