Explanation:
The
matter in galaxy cluster 1E 0657-56,
fondly
known as the "bullet cluster", is shown in
this
composite image.
A mere 3.4 billion light-years away,
the bullet cluster's individual galaxies are seen in the
optical image data, but their total
mass adds up to far less than
the mass of the cluster's two clouds of hot x-ray emitting gas
shown in red.
Representing even more mass than the optical galaxies and
x-ray gas combined, the blue hues show the distribution of dark
matter in the cluster.
Otherwise invisible to telescopic views, the
dark matter was mapped by
observations of
gravitational lensing
of background galaxies.
In a text book example of a shock front, the
bullet-shaped cloud of gas at the right was
distorted during the titanic collision
between
two galaxy clusters
that created the larger bullet cluster itself.
But the dark matter present has not interacted with the
cluster gas except by gravity.
The clear separation of
dark matter
and gas clouds is
considered direct
evidence
that dark matter
exists.
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