For decades the problem seemed to be a symmetry or cancellation
mechanism of some sort that would make the cosmological constant precisely zero. The
single greatest failure of our most promising theories (such as string theories) is that they
do not satisfy this requirement. Now that a dark energy has apparently been found, the
problem is even harder: not just to explain why the dark energy is so tiny compared with
what would have been expected theoretically, but also to explain why it happens to be
of the same order of magnitude (roughly twice) as the energy in matter at the present
moment in the history of the universe.
Hmmm, I wasn't able to post on this after I'd quoted... must have been some sort of "dark matter" problem. That quote was from the first of the many, many citations. What an amazing website that is. When I used to share a computer, those photographs came in the email everyday. They are just astounding.
So Rosm.... do you understand this and can explain more? Are you a physicist?