ACIS cluster montage
Credit: X-ray: NASA/IOTA/S. Allen et al. Optical: HST

Darkness and Light

Most of the Universe is made of stuff we can't see, so-called "dark matter". This material can only be detected by the effect of its gravity on nearby "visible" matter. The images on the right show optical images of 2 clusters of galaxies. The visible light is dominated by the combined starlight from the individual stars in the member galaxies (along with bright arcs produced by gravitational lensing of distant objects by the mass in the galaxy cluster). The images on the left are X-ray images of the same clusters obtained by the Chandra X-ray observatory. the X-ray images trace the distribution of the dark matter. If our eyes were sensitive to dark matter, the galaxy clusters would appear similar to their appearance in the X-ray images. In most clusters of galaxies, most of the mass is comprised of "dark matter"; only a small percentage of the matter in these clusters is visible.

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Page Author: Dr. Michael F. Corcoran
Last modified September 17, 2001