X marks the spot in dark matter web

 作者:呼延红     |      日期:2019-03-14 04:18:06
We cannot see the streams of dark matter forming a web across the sky, but unusual cross-shaped galaxies may mark where they intersect. Unlike the Milky Way’s single disc, “polar ring” galaxies have two discs of stars and gas aligned nearly perpendicular to each other. It is unclear how they form, though one theory suggests they are the product of collisions between two galaxies. But galactic pile-ups are violent and messy, so are unlikely to produce the neat symmetry of a polar ring galaxy,