This illustration shows how the Hubble Space Telescope searches for missing ordinary matter, called baryons, by looking at the light from quasars several billion light-years away. Imprinted on that light are the spectral fingerprints of the missing ordinary matter that absorbs the light at specific frequencies (shown in the colourful spectra at right). The missing baryonic matter helps trace out the structure of intergalactic space, called the "cosmic web".

Credit: NASA, ESA, and and A. Feild (STScI)