When it comes to the Universe, it mainly generates X-rays through high-temperature heating.
When matter heats up, through collisions, interactions, acceleration or collapse, it can emit X-rays.
Galaxy clusters, supernova remnants, active galaxies, binary star systems, and even the Moon emit them.
Yet the most important X-ray image of all time was an incredible surprise.
This is the Bullet Cluster: a system of two galaxy clusters colliding at high speeds.
As the gaseous matter inside collides, it slows, heats up, and lags behind, emitting X-rays.
However, we can use gravitational lensing to learn where the mass is located in this system.
The bending and shearing of light from background galaxies shows it’s separated from the matter’s and X-rays’ location.
This separation is some of our strongest evidence for dark matter.
Since then, over a dozen additional colliding clusters display such a separation, in a variety of configurations.
Whatever dark matter is, it cannot be accounted for by the Universe’s normal matter alone.
The Bullet Cluster images were the first to demonstrate this effect.
NASA’s Chandra, which took the image, has been rightfully renewed as NASA’s flagship X-ray observatory after 19 continuous years.