I am 9 years old. Can anyone tell me the answer on this one?
What do you mean? The mass per particle? Not known. In fact, not much is known about dark matter, except that it doesn't interact electromagnetically, (strongly, or weakly, either, I think?), but it does interact via gravity.
Just like normal matter, you would have to tell us a quantity for us to give you a number for mass. Since we don't even know what dark matter is we cannot even say that it is a certain mass per particle. (If it is even made out of particles)
Hi quanum12345, that's a great question! Many scientists are working very hard to figure the answer out. What we know is that there is at least about 5 times as much dark-matter mass, as 'regular' (called, "baryonic") mass. Also, for a number of reasons, we expect that the mass of each dark-matter particle is much-much more massive than any of the particles which make up normal matter (i.e. protons, neutrons, especially electrons).
Many of the candidates for the particles which make up dark matter are called WIMPS, which stands for "Weakly Interacting Massive Particles". They are 'weakly interacting' because they only interact gravitationally (in-other-words, no electricity-and-magnetism which is the primary way day-to-day objects interact); and 'massive particles' because we think they are so much more massive than protons (for example).
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