As noted below, we are missing a good dark matter candidate.
What would one look like that would somehow fit the Standard Model?
1. Doesn't interact via the weak force.
2. Lower in mass than electrons, heavier than neutrinos.
3. Does interact via the electromagnetic force with a charge of +/- 1/2.
4. Fermionic with spin 1/2 rather than bosonic with spin 1.
5. Does not contribute to large scale electromagnetism, perhaps due to generally being found in a charge neutral composite particle.
Of course, a W boson would seem just the thing for converting a charge +1/2 version into a charge -1/2 version, but then we'd have to figure out how to get precision electroweak measurements to allow that given W/Z boson decay universality.