a halo darkly

Via the always interesting Kottke (who’s sporting a spiffy new look) we learn that tiny, dwarf galaxies as they age can attain a higher concentration of dark matter than the apparent universal constant of one part to five (in favour of the weakly reacting sort of matter rather than the one, baryonic, that we experience) because normal matter is much more flightly and prone to erosion by more massive galactic neighbours.
As counterintuitive as it seems, that much was at least expected and could be a sort of lens for getting to better understand the unidentified nature of dark matter, but astronomers were astounded to detect a wholly new class of galaxies—first spotted in the faint and shy wall-flower called Dragonfly 44. The galaxy appears to be almost entirely composed of dark matter (whether there’s a totality of dark energy as well in the mix is not mentioned) but it’s roughly the same size as the Milky Way. Imagine a galaxy our size but instead of four-hundred billion, there’s only a paltry four to eight billion stars knocking together out there—suggesting that there is something very flawed in the way we think galaxies coalesce and evolve, since those lonely stars ought to be pulled apart and absorbed into other star systems. Researchers are hoping to find more of these big, “empty” galaxies looming closer to home and perhaps observe dark matter and its properties directly.