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The new cosmic photo, a wide-field view from the European Southern Observatory, reveals many thousands of distant galaxies, including a large group belonging to the massive galaxy cluster known as Abell 315.
This wide-field, deep image reveals thousands of galaxies crowding an area on the sky roughly as large as the full moon. Beginning in the centre of the image and extending below and to the left, a concentration of about a hundred yellowish galaxies identifies a massive galaxy cluster, designated Abell 315, about two billion light-years away from us. Full Story. Credit: ESO/J. Dietrich
In these clusters, the galaxies themselves contribute to only 10 percent of the mass, with hot gas in between the galaxies accounting for another ten percent. The remaining 80 percent is made up of an invisible and unknown ingredient called dark matter that can be found in between the galaxy structures.
The presence of dark matter is revealed through its gravitational effect: the enormous mass of a galaxy cluster acts on the light from galaxies behind it (much like a cosmic magnifying glass), bending the trajectory of the light and thus making the galaxies in the background appear slightly distorted.