Well, fast is a few hundred million years. That's fast compared to the age of the universe.

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We have found that, though their stars are generally old, the galaxies that result from these mergers are relatively young.

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This event could be so powerful that it could cause ripples in space time.

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It shows that these galaxies are not in equilibrium, that something is pulling stars out of these galaxies. On this scale the only thing we know of that can do this is another galaxy, and we can actually see that.

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Or, at a previous point the central black hole created so much energy that it pushed the gas out of these galaxies. That is what's next on the agenda to figure out.

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The Milky Way will collide in the future, in about 4 billion years with the galaxy Andromeda and that collision will lead to formation of a much bigger galaxy. So we have that to look forward to.

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