Q: Why is dark matter considered a particle?

A: Actually, nobody has any idea if it is a particle or not. It might be a field.

Dark Matter is a concept used to try to explain why stars in a galaxy seem to be attracted to each other a lot more than their mass would indicate they should.

Given the speed of rotation of galaxies, combined with estimates of the masses of stars and what their gravitational attraction should be over the distances they are from each other, they should be flying apart, but they don't... they keep swirling in a spiral.

Other explenations of the phenomena includes:

a) Maybe the black hole at the center of the galaxy is a womper that's a lot bigger than people thought,

b) Maybe there's a lot more little black holes sprinkled thoughout the galaxy like teeny tiny raisins in a pudding, all adding their powerfull gravitational force together to make a sort of gravitational glue for the galaxy,

c) Our estimates of how much matter there is in the galaxy are largely drawn from looking at stars, which we can see because they are lit up, but maybe there's a lot more ordinary matter in the form of planets and dust that never congealed into a ball big enough to light up as a star to be seen than we thought.

etc.

Q: What properties does a particle have?

The first thing they must have to be called a particle is momentum.

And actually, that's about all they have to have to be called a particle.

Usually they have spin, and often they have charge.

If they've got charge, it can be combinations of the elctromagnetic charge, and/or the colour (stong nuclear) charge, and/or the charge of the weak nuclear filed (expressed by particles of exchange called the W and Z bosons).

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