Large galaxy clusters are typically embedded in a cloud of ionized gas at a temperature of roughly 100 million K. At these temperatures, many of the electrons are moving at more than escape velocity, so do all the electrons run off?
Some do, of course, leaving the cluster with a net positive charge. But what the combined gravitational force of tens of trillions of solar masses of stars, gas and dark matter cannot do, the combined unbalanced electrical force of a few kilograms of electrons finds easy peasy. Electrons escape until the unbalanced electrical force holds them bound. A similar effect results in a net charge for the Sun.
X-ray emission from Abell 1689. The X-Ray emission comes from the hot (10^8 K or so) plasma that constitutes 1/2 or more of the baryonic mass of the cluster.