Most of the matter in the Universe consists of stuff we can’t see. It is dubbed “dark matter” and we know it must be out there. Without dark matter rapidly spinning galaxies (such as those circled, above) would not have sufficient gravitational glue to hold their stars and gas clouds together. These elements would fly off into space instead, like rain drops on a spinning bicycle wheel. What might this ghostly, galaxy glue be made of? Nobody knows. But in 2006 astronomers got a new clue.
X-rays of a bullet cluster
In 2006, NASA astronomers aimed their orbiting Chandra X-ray Observatory at the galaxy cluster shown on the previous image, 1E 0657-56. It captured a very different picture.
Chandra picks up the X-rays given off by hot clouds of gas (shown above in red, overlaid on a Hubble snapshot). The striking shape of the newly revealed gas clouds earned them an instant nickname: the bullet cluster.
But the bullet cluster had a bigger secret to reveal.
When galaxies collide
Astronomers think the bullet cluster began to form around 100 million years ago, when one small cluster of galaxies barrelled right through the middle of a larger cluster, and out the other side. The four-step artist’s illustration above depicts the sequence of events.
The colliding gas particles in each galaxy are shown in red. As the little gas cloud elbowed its way past its bigger partner it acquired its dramatic bullet shape.
Dark matter revealed
What about the dark matter?
Astronomers can track the location of dark matter because its gravity bends the light of stars behind it. The technique is called “gravitational lensing”. Using the Hubble Space Telescope, they were able to see where the dark matter was located in the bullet cluster (violet shading).
While the gas particles jostled and elbowed their way past each other, the dark matter particles slipped right past unnoticed – just what you’d expect from ghosts.
Ghost map
It may be ghostly but the Hubble telescope can detect dark matter because of the way it bends light from stars. Using this information, in 2007 astronomers mapped its location in the Universe and how it has changed over billions of years (pictured).
In the early days of the Universe (far right), dark matter was spread out quite evenly. But over time, gravity collapsed this structure into dense clumps (far left).
Some astronomers think these dark matter clumps created an essential scaffold. Ordinary matter was drawn to it and started forming stars, galaxies and ultimately, ourselves.