Davidsen and Bi appear to have found the normal matter out between the galaxies. Furthermore, the amount they find is completely consistent with the amount we expected to be there from our nuclear physics arguments, so the whole picture holds together remarkably well. Visible matter is seen in the form of stars and galaxies, which emit light and other forms of radiation. Dark matter has not been seen directly, but it is inferred to exist from the gravitational effects it appears to exert on the visible matter. Dark matter itself appears to come in at least two varieties.
Baryons are ordinary matter particles like protons and neutrons.
But astronomers have not been able to account for all the baryonic matter that is thought to exist, based on their studies of the nuclear reactions that occurred during the Big Bang. The visible stars and galaxies contain only a small fraction of the total amount of such ordinary baryonic matter believed to exist. The other component of the dark matter is widely believed to be some sort of exotic particle that does not emit or absorb light. The analysis reported in the Hopkins paper suggests that the missing baryonic matter has been found. It was spread throughout intergalactic space in the form of a very diffuse gas of hydrogen and helium atoms whose presence is detected through its effects on light passing through it.