So, dark matter is not made up of baryons. What could it be made up of? One suggestion is that baryonic matter could actually be a candidate if it were tied up in brown dwarfs, or small, dense chunks of the heavier elements. These are known as massive compact halo objects, or MACHOs.
A view that is more common than this suggestion is that baryonic matter has nothing to do with dark matter at all. It might be made up of other, more exotic particles such as axions, or 'weakly interacting massive particles' - WIMPS.