When we look up to the heavens on a clear night, we see an immense dark sky with uncountable stars. With a small telescope we can also see galaxies, nebulae, and the disks of planets. If you look at the sky with an X-ray detector, you would see many of these same familiar objects; in addition, you would see the whole sky glowing brightly with X-rays.
This glow is called the “diffuse X-ray background.” While, at higher energies, the diffuse emission is due to point sources too far away and faint to be seen individually, the origins of the soft X-ray glow have been controversial, even 50 years after it was first discovered. The long-standing debate centers around whether the soft X-ray emission comes from outside our solar system, from a hot bubble of gas called the local hot bubble, or whether the emission comes from within the solar system, due to the solar wind colliding with diffuse gas.