What to watch for
Turn the halo off and the total curve collapses onto the teal disc-only line: it rises through the bulge, peaks near one disc scale-length, then sags as . That sag is the prediction from the light alone: and it's wrong by a factor of two in the outskirts of real galaxies.
Turn the halo on and watch the outer curve lift and level off. The stars in the galaxy panel respond too: with the halo, the outer stars sweep around far faster than their luminous gravity could ever manage. Nudge the halo scale to feel the trade-off: a compact halo flattens the curve early but contributes little at the very edge; an extended one keeps lifting the curve all the way out. There is no single dial that mimics this with visible matter only, which is exactly why the missing-mass problem refused to go away.
Knobs
- Disc mass: the total mass of the luminous, baryonic disc (stars and gas). More disc raises and broadens the peak, but its curve always eventually declines.
- Halo mass — the amount of dark matter. Set it to zero to recover the pure Keplerian-declining baryonic prediction; raise it to flatten and lift the outer curve.
- Halo scale — the core radius of the dark-matter halo (in kiloparsecs). Small values concentrate the halo near the centre; large values spread it out so it dominates the far outskirts.
- Dark-matter halo — toggle the halo on or off. Off shows what the visible galaxy predicts on its own; on shows what we actually see.