Thousands of galaxies crowding an area on the sky roughly the size of the full moon have been captured in a new photo released today.

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.

Yet, as crowded as it may appear, this assembly of galaxies ? like most galaxy clusters ? is dominated by dark matter that remains unseen. And while the actual existence of dark matter remains largely unexplained, this mysterious stuff has helped scientists piece together other parts of the cosmic puzzle. For instance, dark matter's gravitational pull on galaxy clusters helped researchers calculate the mass of Abell 315.

When stargazers scan the night sky with the unaided eye, they mostly see only stars within our own Milky Way galaxy and some of its closest neighbors. More distant galaxies tend to be too faint to be perceived by the human eye, but if they could be seen, they would literally cover the entire sky.

These galaxies span a vast range of distances. For those that are relatively close, it is possible to distinguish their spiral arms or elliptical halos, particularly in the upper part of the image.

The more distant galaxies appear much fainter. The light from these galaxies have travelled through the universe for 8 billion years or more before reaching Earth. The universe is about 13.7 billion years old.

The thing about Abell (315)

The galaxy cluster Abell 315 can be seen in the image beginning in the center and extending below and to the left. Abell 315 ? so-called because it was designated with the number 315 in the catalogue compiled by the American astronomer George Abell in 1958 ? is a concentration of about a hundred yellowish galaxies.

The galaxy cluster is located about 2 billion light-years away from Earth in the constellation of Cetus (the Whale).

Galaxy clusters are some of the largest structures in the universe held together by gravity, and while these types of images can reveal a rather crowded field of view, there is actually more in these structures than what we see. ?

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.

Dark matter secrets

By observing and analyzing the twisted shapes of these background galaxies, astronomers can infer the total mass of the cluster responsible for the distortion, even when this mass is mostly invisible.

Still, this effect is tiny, and to be accurate, it is necessary to measure it over a huge number of galaxies to obtain significant results. With Abell 315, the shapes of almost 10,000 fainter galaxies in this image were studied in order to estimate the total mass of the cluster, which amounts to over a hundred thousand billion times the mass of our sun.

A handful of other objects that are closer to Earth and much smaller than galaxies and galaxy clusters are also scattered throughout the image, including stars belonging to our own galaxy, as well as many asteroids that are visible as blue, green or red trails. These objects belong to the main asteroid belt, located between the orbits of Mars and Jupiter.

The image was taken by the Wide Field Imager on the MPG/ESO 2.2-meter telescope at ESO's La Silla Observatory in Chile.