Is the gravity of the above galaxies high enough to contain the glowing
hot gas? Superposed on an optical picture of a group of galaxies is an image
taken in X-ray light. The X-ray picture, taken by ROSAT, shows confined hot
gas highlighted in false red color, and provides clear evidence that the
gravity exerted in groups and clusters of galaxies exceeds all
the individual component galaxies combined. The extra gravity is attributed
to dark matter, the nature and abundance of which is the biggest mystery
in astronomy today.