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.