Clouds of neutral hydrogen are ubiquitous in the Milky Way galaxy, and effectively absorb photons that are energetic enough to ionize hydrogen, which requires an energy of 13.6 electron volts (in the extreme ultraviolet range). Even the relatively small amounts of hydrogen in the Lockman Hole absorb most radiation at and just above energies of 13.6 electron volts, but even so it transmits extreme ultraviolet and soft x-ray radiation from extragalactic objects to a greater degree than other areas of the sky.