X-rays are high in energy but have short wavelengths.

X-rays from space interact with the oxygen and nitrogen in the Earth's atmosphere. Due to the interaction, they are absorbed before reaching the surface of the Earth.

Also, due to their short wavelengths they pass right through traditional mirrors used to focus visible (light) waves.

Astronomers must build special detectors to observe X-rays. They place these detectors above the Earth's atmosphere.