Philosopher Andrew Sepielli offers a kind of precis of his book on the subject.  He writes:

Morality is objective, but it neither requires nor admits of a foundation. It just kind of floats there, along with the evaluative realm more generally, unsupported by anything else. Parts of it can be explained by other parts, but the entirety of the web or network of good and evil is brute. Maybe you think that’s weird and even worthy of outright dismissal. 

I'm still in the "that's weird and even worthy of outright dismissal" camp, but his very readable essay is worth a look!