New results gathered from observations of galaxy clusters with Hubble and the Very Large Telescope have shown that the Universe — and this is a consistently irritating property of it — isn't performing as our physics thinks it should.
In this case the focus is on dark matter. When you look at all the matter in the Universe, what we think of as "normal" matter — stuff made up of electrons, neutrons, protons, and so on — is in the distinct minority. Only about one-sixth of all matter is normal; the rest is dark matter.
We don't know a lot about dark matter, except that it gives off no light (hence the name) and doesn't interact with normal matter except through gravity. But we know it exists. We see galaxies, for example, rotating differently than expected given their shape and amount of normal matter. The only way this can be is if they are surrounded by haloes of dark matter, whose gravity gives the stars and gas in the galaxy an extra tug, affecting the way the stars move.
The galaxy cluster S1063 is a collection of hundreds of galaxies and is strewn with dark matter. This acts as a ...