Read more: 鈥�Mysteries of the Milky Way鈥�
Pacing out their slow circuits over a billion years or more, 26 small galaxies are known to orbit the Milky Way. That might seem an impressive band of loyal followers. Astrophysicists, though, think we should have an army.
That expectation is based on the prevailing model of how dark matter helps to form galaxies. Dark matter鈥檚 composition is unknown, but it is thought to outweigh ordinary matter by five to one. In simulations of the early universe, the gravitational pull of cold clumps of dark matter draws in ordinary gas to form the first galactic building blocks.
Advertisement
The theory works well on large scales, reproducing the spongy pattern of galaxies and voids seen across the cosmos. On smaller scales, however, the simulations show that around every large spiral galaxy, dark matter clumps should sculpt thousands of dwarf galaxies.
One possible explanation for the discrepancy is that dark matter is not cold and clumpy, but a hotter gas of lightweight particles that forms small blobs less easily. Or perhaps dark matter doesn鈥檛 exist at all: if the strength of gravity were to change at long range, that would do much that dark matter does without requiring so many dwarfs.
Tinkering with gravity is controversial. A less radical idea is that all those small dark-matter blobs do exist, but we just can鈥檛 see them. Because their gravitational grip is weak, the gas could have been pushed out before many stars could form; a few giant stars may have blasted it out with their fierce heat and explosive deaths, for example. This theory is hard to check in simulations, says astrophysicist of the University of Oxford, because it depends on details such as the local gas density. 鈥淭hat has very fine structure which is way beyond the resolution of any simulation.鈥�
If the idea is right, though, it has a startling implication. Thousands of dark galaxies are arrayed around the Milky Way: a legion not lost, but invisible.