This elusive form of matter is one of the most frustrating and mysterious aspects of modern astronomy. Thought to account for 80% of all matter in the universe, it is completely invisible, detectable only through its slight gravitational pull on its surroundings.
But in some situations, it can settle into the core of a massive object, releasing energy in the form of heat. Now, a pair of astronomers is advocating a daring new research program: to turn our widening search for life beyond Earth into a hunt for dark matter.
We know very little about dark matter, other than that it exists. In the 1970's, astronomer Vera Rubin noticed something funny about the way that galaxies were rotating. Rubin found that stars were orbiting around their galaxies far too quickly, given how much visible matter there was if you add up the gravitational attraction of everything we can see in a galaxy, then at the observed rotational speeds the galaxies she trained her telescope on should have torn themselves apart billions prior to her observation.
In the decades since Rubin's revelation, more mysteries have piled up. The gas inside galaxy clusters is too hot. Galaxies move around too quickly. The universe has too many large-scale structures, given the age of the universe. The remnant radiation from the early universe is too bumpy to be explained by normal matter alone. Light from distant background galaxies curves too strongly when passing near massive galaxy clusters.
The list goes on, but one answer has risen to the top: In order to explain all these observations, the universe must have some hidden ingredient. It's a form of matter (because it can obviously clump together and has gravity), but it doesn't interact with light or normal matter in any significant way. It's dark matter.
Based on computer simulations of giant clusters of galaxies, whatever the dark matter is, we expect it to be more heavily clumped toward the centers of galaxies and generally thin out the farther you get from those centers. And it's those differences in dark matter density throughout a galaxy that may help astronomers identify this mysterious substance.
If only we had large dark matter detectors scattered through the galaxy.