Question

galaxies often exist within galaxy clusters. what holds these clusters together?
o gravity
o galaxy clusters are not bound together
o hydrostatic equilibrium
o electromagnetic attraction

most of the mass in galaxy clusters is in
o giant elliptical galaxies.
o dark matter.
o high - temperature plasma between the galaxies.
o dwarf spheroidal galaxies.
o the brightest cluster galaxy.

Explanation:

Galaxies in clusters are bound by gravity, a fundamental force that attracts massive objects to each other. The majority of mass in galaxy - clusters is in dark matter, which provides the gravitational pull necessary to keep the cluster together and influences the motion of visible matter.

Answer:

First question: gravity
Second question: dark matter