AS IT is, dark matter is stuff we cannot see. So astronomers were stumped when some of it went missing two years ago. Now it has turned up right where it’s supposed to be, much to the relief of those who thought they might have some explaining to do.
The existence of dark matter was suggested in the 1970s when astronomers found that the gravity of visible matter alone was not enough to keep the fast-moving stars and gas in spiral galaxies from flying out into space. They attributed the extra pull to dark matter, which is now thought to…
