To me, the notion of (substantive) universals always conjures some Platonic notion of, say, “redness” as some essence/substance that imbues those things that we see as red – as opposed to some generalization about things that we just see as "red". And as opposed to the fact that experiencing things as "being red" in color simply enables us to generalize to the idea of redness (really a certain wavelength range in the color spectrum, to which we apply the word “red”).
If, as Nate points out, “the only real existences are atoms and void [i.e., the universe is strictly physical in nature], it follows that no abstractions exist” – then, to the extent that they are (and I think they are) abstractions, universals such as “redness” are not themselves any kind of existent.
I wonder if this generalizing aptitude is related to Epicurean prolepsis? If I say I saw a red rose, you have (based on your own memoried experience) an immediate, general grasp of what I mean – without actually seeing the particular rose?