When the ultraviolet light from hot stars in very distant galaxies finally reaches us, it arrives at Earth in the form of visible light.
Explanation:
Astronomers use these three images from the Hubble Space Telescope to find why distant galaxies have such strange shapes that are significantly different from typical elliptical and spiral galaxies in a nearby universe. The light from these kind of galaxies passes long distances (billions of light years) to attain Earth.
When travelling light is "stretched" as the space expansion. As a result, the light is no longer visible, but switched to infrared, where current instruments are less sensitive. The only observable light astronomers are regions inhabited by young hot stars. These stars usually emit ultraviolet light. But this light is stretched and appears as visible light, when it reaches the ground.