As the universe evolves, density fluctuations in the intergalactic gas (hydrogen and helium) grow into clumps and filaments under gravity's pull. The densest clumps form galaxies and quasars whose radiation energizes (ionizes) the remaining intergalactic gas.
Observations using NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) reveal the structure of ionized helium that traces the lowest density regions of the intergalactic medium.
Image Type: Illustration
llustration Credit: A. Feild, F. Summers and G. Kriss (STScI)