Superposed on an optical picture of a group of #galaxies is an image taken in #Xray light.

The X-ray picture, by #ROSAT, shows confined #hotgas 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 #darkmatter.

#APOD classics: https://apod.nasa.gov/apod/ap950902.html