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Name: Stuart Briber
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Assuming the force that holds spinning galaxies together comes largely from the gravity of dark matter, why is it believed this dark matter forms haloes around the OUTSIDE of a galaxy where its gravity would contribute to pulling a galaxy apart?

1. The halo is present inside as well.

2. The existence of dark matter beyond the visible extent of galaxies is inferred by looking at velocities of HII regions seen beyond the main galaxy and are believed to be bound to the galaxy (gravitationally bound).

3. Another evidence for this comes from gravitational lensing of distant objects.

4. Halo outside a galaxy does not pull a galaxy apart, this follows from a well known theorem.

Jasjeet S Bagla

The previous answer is basically correct: the "Dark Matter" is believed to be present throughout the entire galaxy. It extends beyond the visible galaxy's edge, however. Point 4 in the previous response deserves a little more explanation. It is only true if the mass distribution is spherical. If the halo is spherical, as is believed to be the case, the force felt by any particle only depends on how much material lies interior to that particle's position.

Richard A Gerber

Nature magazine had 2 nice articles on the "dark matter" problem. One was a review paper. The other was a measurement in dwarf galaxies that made an impressive case against "cold" dark matter. I think that the upshot is that almost all dark matter could be hadronic (and the universe can only be closed by Einstein's fiddle-factor).

J Lu

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