Estimates of the total amount of dark matter in the universe start with estimates of the total amount of matter in the universe via methods like relativistic lensing and then subtract out known sources of ordinary matter from observed or easily inferred sources. But, the ratios of dark matter to all matter in the universe commonly quoted in scientific journalism directed at the general public does not reflect these multiple new discoveries. Adjusted for these new discoveries dark matter accounts for only about half of all observed matter.
Equally notably, the latest research shows that the vicinity of galactic clusters is highly atypical of the regions on the fringes of ordinary galaxies, having far more "dim matter" than ordinary galaxies. Theories like MOND, which estimates of the nature of the effects usually attributed to dark matter based on empirical correlations observed between rotation curves and luminous matter distributions are usually remarkably accurate even with just a single parameter to be fit to the data and do so without requiring the existence of some class of fundamental particle that has been largely ruled out by Earth bound experiments.
In contrast, all existing dark matter theories can only fit the data from ordinary galaxies with multiple parameters and require some, as yet undiscovered, new kind of fundamental particle that is not simply an ordinary atom. Dark matter theories also have a far weaker track record when it comes to making predictions about not yet observed types of galaxies.
But, these modified gravity theories greatly underestimate the observed effects in galactic clusters. The large amounts of "dim matter" in galactic clusters may help to explain this discrepency. For example, even more sophisticated version of MOND simply cannot be fit to the Bullet Cluster collision that has been observed.
If galactic clusters have large quantities of dim matter that are absent in the vicinity of ordinary galaxies, in addition to ordinary MOND effects (whether its mechanism is true dark matter, or some predictable distribution pattern of dim matter, or from a modified gravity law), the case for a MOND Plus theory to describe galactic clusters the cures the deficiencies of MOND theories in the domains where they fail, sounds plausible.
The Problem and A Possible Solution
Astronomers disagree about why they see more light in the universe than should be seen; that is, why the infrared light they observe exceeds the amount of light emitted from known galaxies.
When looking at the cosmos, astronomers have seen what are neither stars nor galaxies nor a uniform dark sky but mysterious, sandpaper-like smatterings of light, what UCLA professor of physics and astronomy Edward L. (Ned) Wright refers to as "fluctuations".
The data appear to rule out the possibility that the fluctuations in the infrared background are from very distant unknown galaxies and also rule out the possibility that the fluctuations "have been traveling to us from faint galaxies for only 4 or 5 billion years[.]"
So what could be going on? According to Wright:
"Galaxies exist in dark matter halos that are much bigger than the galaxies; when galaxies form and merge together, the dark matter halo gets larger and the stars and gas sink to the middle of the the halo. . . .What we're saying is one star in a thousand does not do that and instead gets distributed like dark matter. You can't see the dark matter very well, but we are proposing that it actually has a few stars in it — only one-tenth of 1 percent of the number of stars in the bright part of the galaxy. One star in a thousand gets stripped out of the visible galaxy and gets distributed like the dark matter. . . . The dark matter halo is not totally dark. A tiny fraction, one-tenth of a percent, of the stars in the central galaxy has been spread out into the halo, and this can produce the fluctuations that we see."In addition to having hig percentage of stars that are not a part of particular galaxies, galactic clusters also greatly elevated levels of interstellar gas and could also harbor large quantities of neutrinos. Leptogenesis theories suggest that there aren't nearly enough neutrinos in the universe to account for all dark matter, but there are more than enough "missing neutrinos" in the universe to account for excessive amounts of non-baryonic matter in galactic clusters.
In large clusters of galaxies, astronomers have found much higher percentages of intra-halo light, as large as 20 percent, they contend.