Complex sentence: The discovery of intergalactic stars addresses the "missing baryon problem"--baryons being a class of subatomic particles including the protons and neutrons that make up the cores of the atoms in normal matter.
Split into simpler sentences: The discovery of intergalactic stars addresses the "missing Baryon problem". Baryons, being a class of subatomic particles including the protons and neutrons that make up the core of the atoms in normal matter.