The Short, Obvious Answer:
Stars OUTSIDE the galaxies in clusters.

From SDSS "Navigate" Tool

The Long, Interesting Answer:

• The high galaxy and mass densities in clusters lead to strong tidal forces from interactions between galaxies, groups of galaxies, and the overall cluster potential.
• These tidal forces rip the stars from their parent galaxies, leaving behind a diffuse, low surface brightness glow.
• Because the ICL is a direct product of the dynamical history of the cluster and the cluster galaxies, we can use it as a "fossil record" of the cluster's evolution, gaining a window into the processes involved in both galaxy evolution and the assembly of structure in the universe.
ICL Formation Movie

	Stars are stripped from the galaxies through their numerous gravitational interactions, resulting in a transfer of luminosity from high to low surface brightness (i.e. galaxies to ICL).


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