[Lamplighter]

Dark matter: I know it's there ... because I can't see it.



Dark matter 'ghosts' through galactic smash-ups
BBC - 3/26/15

By observing multiple collisions between huge clusters of galaxies,
scientists have witnessed dark matter coasting straight through the turmoil.

Dark matter is the mysterious, invisible stuff that makes up 85% of the matter in the cosmos
- and these results rule out several theoretical models put forward to explain it.
This is because it barely interacts with anything at all, including the dark matter in the oncoming galaxies.
…
"In all of these collisions that we've seen, it just seems to go straight through.
And now we've seen loads more of them, we would have been able to detect any deceleration
of this dark matter, if it had interacted in the ways that most theories predict," Dr Massey said.

So although some theories remain, many can now be ruled out.
This includes the idea that dark matter is some sort of "dark" version of ordinary matter, made of "dark atoms".

It must be more outlandish than that, Dr Massey said.
"Basically, we're saying: Back to the drawing board!
Let's come up with some more ideas."