"Galaxy clusters" refers to groups of galaxies that are bound together by gravity. They are like neighborhoods in space where many galaxies live close to each other. Full definition
A study of galaxy clusters independently confirms that the universe is expanding at an accelerating pace. (kavlifoundation.org)
Dark matter was first proposed more than 70 years ago to explain why the force of gravity in galaxy clusters is so much stronger than expected. (businessinsider.com)
In fact, the colors represent dark matter, galaxies and hot gas formed from the collision of massive galaxy clusters. (livescience.com)