Young galaxies clearing the hydrogen fog from space in the early universe. In the first billion years or so of the existence of the universe, the space was filled with an opaque fog of hydrogen, which interacted with the light emitted by young galaxies. Over time, radiation from the first generation of stars reionised this hydrogen, and the universe became transparent, permitting the transmission of light. This is the first light that is detectable today. In 2010, the discovery of the most distant galaxy, UDFy_38135539, was announced, visible within a bubble of transparent hydrogen. This is the first known object detected from the period of reionisation, and it is thought to have been detected when the universe was 600 million years old.