Scientists were able to measure the cosmic optical background (COB) thanks to the data from New Horizons space probe at the edge of the Solar system. The light is significantly brighter than what light from known sources can explain. According to a new study led Teresa Symons it is two to three times brighter.
Lyman Alpha is a specific line in electromagnetic spectrum, which is emitted when an electron jumps from 2nd orbital state to the 1st one. In inner solar system the Lyman-Alpha radiation from the Sun is prevalent, but beyond the orbit of Pluto, the galactic variety comes to light.