To be clear - you are talking about morality as something externally existing, some 'facts' that exist in the world and dictate what you should do, as opposed to a human system of don't be a jerk. Is that an accurate portrayal?

If that is the case, there are two big questions that immediately come to mind (beyond "what are these facts" and "where did they come from") - first, it seems that Moral Facts would have to interact with the world in some way in order for the study of big-M Morality to be useful at all (otherwise we could never learn what they are), or they would have to be somehow deducible from first principles. Are you supposing that they somehow directly induce intuitions in people (though, not all people? so, people with certain biological characteristics?)? (By (possibly humorous, though not mocking!) analogy, suppose the Moral Facts were being broadcast by radio towers on the moon, in which case they would be inaccessible until the invention of radio. The first radio is turned on and all signals are drowned out by "DON'T BE A JERK. THIS MESSAGE WILL REPEAT. DON'T BE A JERK. THIS MESSAGE WILL...".)

The other question is, once we have ascertained that there are Moral Facts, what property makes them what we should do? For instance, suppose that all protons were inscribed in tiny calligraphy in, say, French, "La dernière personne qui est vivant, gagne." ("The last person who is alive, wins" - apologies for Google Translate) Beyond being really freaky, what would give that commandment force to convince you to follow it? What could it even mean for something to be inherently what you should do?

It seems, ultimately, you have to ask "why" you should do "what you should do". Common answers include that you should do "what God commands" because "that's inherently What You Should Do, it is By Definition Good and Right". Or, "don't be a jerk" because "I'll stop hanging out with you". Or, "what makes you happy and fulfilled, including the part of you that desires to be kind and generous" because "the subjective experience of sentient beings are the only things we've actually observed to be Good or Bad so far".

So, where do we stand now?