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The Milky Way Is Much Less Massive Than Previous Thought
schwit1 writes: New research by astronomers suggests that the Milky Way is about half as massive as previously estimated. It was thought to be roughly the same mass as Andromeda, weighing in at approximately 1.26 x 10^12 solar masses (PDF). This new research indicates its mass is around half the mass of Andromeda. "Galaxies in the Local Group are bound together by their collective gravity. As a result, while most galaxies, including those on the outskirts of the Local Group, are moving farther apart due to expansion, the galaxies in the Local Group are moving closer together because of gravity. For the first time, researchers were able to combine the available information about gravity and expansion to complete precise calculations of the masses of both the Milky Way and Andromeda. ... Andromeda had twice as much mass as the Milky Way, and in both galaxies 90 percent of the mass was made up of dark matter."
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Check out these before and after shots: you can't even see the dark matter anymore.
Dust blocks light and turns it into heat, which it radiates. So it shows up on infrared telescopes you mentioned.
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Marketing suggested that now it should be called the Skim Milky Way.
All known stable forms of matter take only four forms: The lightest charged lepton, the electron, the lightest baryon, the proton, the nearly massless neutrinos, and atomic nuclei built from bound states of protons and neutrons. All but the neutrinos interact with matter in well-known ways that we can definitely measure (they are, or contain, electrically charged particles and therefore emit and scatter photons, which are relatively easily picked up). The neutrinos only undergo weak force interactions, but modern detectors can still pick them up. Their known number density and the limits on their mass mean they can't account for the observed gravitational binding of galaxies.
Any normal matter would interact with the light from objects behind it. This is the origin of effects such as the Lyman Forest which reveals the distribution of cold hydrogen in the flight path of distant light... The bottom line is, baryonic matter in the quantities implied simply has nowhere to hide. It's just too easy to detect ten times a galaxy's mass of matter sprinkled among the galaxy's visible part.
Hence the hypothesis of a particle which has mass but no electric charge, color charges or weak hypercharge. It would cast a gravitational shadow but otherwise be virtually undetectable since it does not undergo any interactions we can make individual particle measurements of. Hence the 'dark' in dark matter. One of the few alternatives to a dark particle is a universe suffused with low-mass black holes, but the lack of either microlensing events or gravitational waves emitted by their scattering off each other is difficult to explain. There are a few other places in GR that you can insert hypothetical terms without making it blow up in the face of observations - in fact Einstein's original cosmological constant very effectively explains accelerating expansion. It's also possible that GR isn't the correct theory of geometrodynamics, and the effect of higher-order curvature terms or such is not zero.
GR and QFT are fundamentally incompatible (GR is classical, QFT is quantum), so while there absolutely must be new physics out there, the question is where the new physics lays and what form it takes (and can we ever reach the energy levels to directly investigate it). The belief among physicists is that the correct theory should be the simplest one which fully explains observed phenomenon - Hence why, for example, GR as currently postulated does not involve any higher order curvatures - and the standard cosmological constant / cold dark matter framework does a remarkable job of explaining the evolution of the universe to its present state with remarkably few parameters.