If you're a researcher at Cambridge University's Institute of Astronomy in England, the answer is: Use painstaking calculations and a really big telescope.
With the help of data from the world's most advanced optical array, Cambridge scientists announced last week that they may be a step closer to identifying properties of dark matter.
This mysterious yet pervasive substance could be the "cosmic glue" that keeps rotating galaxies from spinning apart.
"Pretty much nothing had been known about dark matter, " said Gerry Gilmore, a professor of experimental philosophy and lead researcher on the project.
"All that we knew was that it was transparent and it was heavy. We knew it had weight because it's what holds stars in the sky; without it they'd all fly off into space."
Now the researchers report that, according to their calculations, dark matter travels at around six miles (nine kilometers) per second.
And if it were made of hydrogen atomshydrogen being the universe's most abundant elementits temperature would reach 10, 000° C (18, 000° F)hotter than the surface of the sun.
Abundant but Elusive
Dark matter is thought to make up almost a quarter of the universe, making it six times more abundant than visible matter, such as planets, stars, and cosmic gases.
The existence of this theoretical substance was first proposed in the 1930s by Swiss astrophysicist Fritz Zwicky.
By studying the rotation of a group of galaxies called the Coma Cluster, Zwicky calculated that the visible mass of the galaxies was 400 times less than the mass needed to explain their gravitational motion.
What is dark matter in the universe?
Dark matter is matter of an unknown type. It is known to exist, due to its gravitational influence, but it is not known what it is made of. There is at least 5 times as much dark matter than "normal" matter.