Clusters are where galaxies are more common … but the image above does not show such a cluster. Hubble is a single telescope equipped with several cameras. Each camera sees a slightly different part of the sky when Hubble is pointed at a target. Two main cameras were utilized for Frontier Fields: the Advanced Camera for Surveys (or ACS), and the Wide Field Camera 3 (WFC3).
For this image, the cluster Abell 2744 (also called Pandora’s Cluster) was the main target, and was being observed by WFC3. At the same time, ACS was viewing the nearby part of the sky shown above.
As you can see, it’s filled with galaxies. I scanned the full resolution 6,750 x 6,500 pixel image, and only found a dozen or so objects I could unambiguously identify as stars (point sources like stars get those cross-shaped diffraction spikes through them; extended objects like galaxies smear out the spikes so they become invisible).
In other words, for all intents and purposes, every object you see in that photo is a galaxy, each a sprawling collection of gas, dust, and countless billions of stars, each thousands or even hundreds of thousands of light years across.