Theoretical Predictions for the Inner Dark Matter Distribution in the Milky Way Informed by Simulations

The Big Picture

Picture searching for something invisible in a pitch-black room packed wall-to-wall with furniture. That’s roughly what astronomers face when trying to map dark matter at the center of the Milky Way.

The galactic center is a brutal place to do astronomy. Dust blocks visible light, billions of stars blur together in tight clusters, and ordinary matter dominates so thoroughly that any dark matter signal nearly vanishes beneath it.

Dark matter makes up about 85% of all matter in the universe, yet no one has detected it directly. We know it exists because its gravity shapes galaxies, bends light, and drives cosmic structure. But where exactly it sits deep inside our own galaxy remains unclear.

That uncertainty has practical consequences. Searches for signals from dark matter annihilation or decay depend on knowing how densely dark matter is packed into the galactic center. Get the density wrong by a factor of 10, and the predicted signal shifts by a factor of 100.

Abdelaziz Hussein and Lina Necib at MIT took a different tack. Tracking individual stellar motions can barely constrain the dark matter distribution within about 20,000 light-years of the center. Rather than trying to measure it directly, the team bracketed the plausible range. They used four galaxy-formation simulation suites, calibrated against what we actually observe about the Milky Way’s stars, to set upper and lower bounds.

Key Insight: The physics governing how ordinary matter reshapes dark matter halos (gravitational contraction pulling dark matter inward, explosive stellar feedback pushing it outward) lets the team define a theoretically grounded range for the Milky Way’s inner dark matter profile. Detection experiments now have a physically motivated target window rather than unconstrained guesswork.

How It Works

When stars form in a galactic center, what happens to the dark matter? Two competing forces reshape the dark matter halo, the roughly spherical cloud of dark matter surrounding a galaxy:

• Adiabatic contraction (AC): As ordinary matter piles up at the center, its gravity pulls dark matter inward, producing a dense, steep concentration called a cusp. This is the default outcome when nothing disrupts the process.
• Baryonic feedback: Supernovae, stellar winds, and active galactic nuclei (supermassive black holes in their energetic phase) blast gas outward in repeated bursts. Each burst shifts the gravitational potential, kicking dark matter to larger orbits and carving out a shallower core.

The team analyzed six Milky Way-mass galaxies from four simulation suites: Auriga, VINTERGATAN-GM, TNG50, and FIRE-2. Each implements these processes differently. Three of them (Auriga, VINTERGATAN-GM, TNG50) produce dark matter profiles that closely match the adiabatic contraction prescription of Gnedin et al. 2004. Their stellar feedback isn’t strong enough to disrupt the infall. FIRE-2 is the outlier: its feedback is aggressive enough to puff the inner halo outward.

The ratio plotted above makes this concrete. In FIRE-2, the dark matter mass in the inner regions consistently falls below the AC prediction; feedback has won. In the other three suites, the ratio hovers near 1.0, meaning contraction dominates. These two outcomes set the upper and lower bounds of the team’s predicted range.

The researchers then plugged in the observed stellar distribution of the actual Milky Way. They didn’t treat simulated galaxies as stand-ins for ours. Instead, they used the simulations to learn the physics and applied that physics to real data.

The predicted inner density slope (how steeply dark matter density rises toward the center) ranges from −0.5 (a shallow, feedback-scoured core) to −1.3 (a steep, contraction-dominated cusp). For reference, the standard NFW profile has a slope of −1 at small radii. The team’s range spans from shallower to slightly steeper than this benchmark.

Halo shape carries its own information. Adiabatically contracted halos are close to spherical, with axis ratios q (1.0 being perfectly round) between 0.75 and 0.9 at 5° from the galactic center. FIRE-2-like halos are oblate, squashed along the disk plane, with q between 0.5 and 0.7. That distinction matters because indirect detection searches typically assume spherical symmetry.

Why It Matters

The J-factor controls how bright an annihilation signal from the galactic center would appear. Across the team’s range, it spans nearly two orders of magnitude: 0.8 to 30 × 10²³ GeV²/cm⁵ averaged over a 5° cone. The D-factor, governing decay signals, ranges from 0.6 to 2 × 10²³ GeV/cm². These aren’t free parameters tuned for convenience. They follow from simulations tied to real stellar data.

The profile shape turns out to be a major source of theoretical uncertainty, one that gamma-ray searches with Fermi-LAT, neutrino surveys, and planned future experiments don’t always fold into their analyses. Hussein and colleagues make the case that distinguishing the two feedback regimes is a prerequisite for interpreting any detection claim or upper limit from the galactic center.

Stellar motion surveys with much greater precision, paired with higher-resolution simulations, could eventually reveal which regime our galaxy actually falls into.

Bottom Line: The Milky Way’s inner dark matter profile remains uncertain by a factor of ~40 in J-factor, but that uncertainty is now bounded by physics rather than parametric freedom. Dark matter hunters have a well-defined target window to work with.