v u lensing events, it is more likely that they are caused by objects in the halo of our galaxy, than the disk.)

It has been suggested that the dark matter could be in the form of neutrinos, if neutrinos have a small rest mass. Remember, we saw in Chapter 9 that there is growing evidence that the neutrino mass is in about the 10~2 eV range This is small enough to have been overlooked in previous experiments. However, there are so many neutrinos in the universe that even a small mass for each neutrino can add a lot of mass to a galaxy. The idea of neutrinos as dark matter in galaxies is still not generally accepted.

The rotation curves of galaxies are our first evidence for the existence of dark matter. We know it is there because we can measure its gravitational effects, but we cannot see it. When astronomers were not sure the matter was there, there was a 'missing mass problem'. However, the mass isn't missing; it is just nonluminous. As we go to larger scales in the universe, we will see that we will find evidence that dark matter becomes more and more important.

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