The biggest things gravity can hold
A galaxy cluster is not one galaxy. It is a giant city of hundreds to thousands of galaxies locked together by gravity, making it the biggest known structure gravity holds together in the universe.Galaxy Clusters: The Universe’s Giant Gravity Cities
Most of it is invisible
The galaxies are only the part you can easily see. A cluster is mostly made of dark matter and gas so hot it glows in X-rays at 30 to 100 million degrees Celsius.Galaxy Clusters: The Universe’s Giant Gravity Cities
