From National Geographi
Image courtesy ESO
Collectively known as NGC 520, the cosmic bodies started colliding about 300 million years ago. Astronomers estimate the merger is now about halfway complete, as the cores of the two galaxies have not yet met.
Published December 2, 2010
Dark matter sounds like some physicist’s tall tale: There’s this invisible matter, see, and it has this powerful gravitational effect on galaxies. That’s why we know it exists. In fact, it outweighs ordinary matter by about five to one. Problem is, dark matter doesn’t reflect or absorb light, so we can’t see it. Oh, and it rarely interacts with conventional atoms, so we can’t feel it, either. However, we know it makes up a huge part of the universe, so we keep looking for it.