What we measure in a dwarf galaxy
A dwarf galaxy has a mass in stars anywhere from about a hundred Suns to a billion Suns. It sits inside a much heavier, invisible cloud of dark matter called a halo. Nobody sees the dark matter directly. Astronomers see its pull: gas and stars circle faster when more mass lies inside their orbit. Measure the orbital speed at each distance from the centre and you get a rotation curve, a running total of the mass enclosed.
Computer simulations with cold dark matter, slow particles that feel only gravity, make halos whose density keeps rising all the way in. That steep centre is called a cusp. Many small, gas-rich galaxies rotate as if their middle had a roughly constant density instead, which astronomers call a core. Bullock and Boylan-Kolchin's 2017 review shows two such galaxies with outer speeds near 40 kilometres per second. The cusp prediction climbs quickly near the centre; the measured speeds climb slowly. Bullock and Boylan-Kolchin 2017, section 2.2
Not every dwarf has a core, and the inner few hundred parsecs are hard to measure. A parsec is about 3.3 light years. The mismatch was still large enough that people asked whether dark matter itself might behave differently.
Four ways to thin out the centre
Take one halo of ten billion solar masses and look 150 parsecs from its centre, about 500 light years out. In the plain cold model, each cubic parsec there holds about a quarter of a Sun's mass of dark matter. The table compares that with three ideas that thin the centre and one that mainly removes small halos. The numbers come from the published formulas used in the companion Lab, for one chosen set of parameters each.
Collisions. If dark matter particles bounce off each other a few times over ten billion years, heat flows from the warmer outer halo into the colder centre. The centre swells into a ball at one even temperature, and its density levels off. The strength of the collisions is quoted as a target area per gram of dark matter. The row above uses two square centimetres per gram, roughly the size of a fingertip for every gram. Kaplinghat, Tulin and Yu 2016
Waves. If the dark matter particle is extremely light, its quantum wavelength stretches across a kiloparsec or so. The centre then settles into one standing lump of wave, called a soliton. Its size shrinks when the particle is heavier. Schive et al. 2014
Gas. Cold dark matter can be thinned by ordinary matter. When stars form in bursts, supernovae push gas out of the centre and it later falls back. The central gravity rises and falls each time, and dark matter orbits drift outward. Read and collaborators found that a core about the size of the stellar body always forms if star formation continues long enough. Read et al. 2016
Warm dark matter is the odd one out. Its particles moved fast in the early Universe and smeared out the smallest ripples, so fewer small halos form and the ones that do are less concentrated. It does not carve a large core of constant density. Bullock and Boylan-Kolchin 2017, section 3.2
Three very different causes put the density at 0.04 to 0.15 solar masses per cubic parsec, against 0.23 for the untouched cusp. With other parameter choices the three can be brought closer still. One rotation curve cannot tell them apart.
Where the ideas part ways
Counting small galaxies. Warm and fuzzy dark matter remove small halos; collisions and gas feedback leave the count of halos almost unchanged. In 2021 Nadler and colleagues compared the Milky Way satellites found by two sky surveys with model predictions. At 95 per cent confidence they found that a warm particle must be heavier than 6.5 keV, and a fuzzy particle heavier than about three times ten to the minus 21 electronvolts. That fuzzy limit is about thirty times heavier than the particle often proposed to explain dwarf cores. A heavier fuzzy particle makes a smaller soliton. Nadler et al. 2021
Small and large halos. In the collision picture, a bigger halo has faster particles and more of them, so its core grows: from about 0.2 kiloparsec at a billion solar masses to nearly 2 kiloparsecs at a hundred billion in the companion Lab's model. A soliton does the opposite and shrinks in heavier halos. Gas feedback follows the stars: it needs enough star formation. Simulations summarised by Bullock and Boylan-Kolchin find little effect once the stellar mass falls below about one ten-thousandth of the halo mass, so the faintest dwarfs should keep their cusps if gas alone is at work. Bullock and Boylan-Kolchin 2017, section 3.1
Galaxy clusters. Collisions strong enough to thin a dwarf would also thin the centres of galaxy clusters, where the particles move a hundred times faster. Measured cluster centres are dense enough to rule out a constant collision strength above roughly half a square centimetre per gram. A collision model that works for dwarfs then needs collisions that weaken as particles move faster. Bullock and Boylan-Kolchin 2017, section 3.2.2 Tulin and Yu review how such speed-dependent collisions arise in particle models. Tulin and Yu 2018
What is settled and what is not
A single cored dwarf does not identify the dark matter. The population does the separating: how many small galaxies exist, how cores change with halo mass, and what the faintest galaxies look like. Satellite counts already push warm and fuzzy dark matter close to cold behaviour on these scales. Simulations still disagree on how small a galaxy gas feedback can core. Whether every observed core comes from gas, or some need new dark matter physics, is an open question.
To follow the physics step by step, start with How a cusp or a core shapes a rotation curve.