... it would best be explained by design.
Design, and the necessary assumption of a designer, explains nothing in any scientific sense. One must then explain the origins of the designer, and the origins of those origins, and so on in an infinite regression. Attempting to terminate that regression by claiming that the designer has always existed begs the question. This is highly unsatisfactory to the scientific mind, which seeks naturalistic explanations of all phenomena.
It does, however, look very much like, as Freeman Dyson put it, that the universe must in some sense have known we were coming. That's the essence of the fine tuning argument that's appeared in other posts here and elsewhere: the constants of nature, like the relative strengths of the four forces, the charge to mass ratio of electrons and protons, and a lot of more esoteric numbers like the nuclear fine structure constant, appear very carefully tuned to permit life as we know it to exist. I've never been sure what the point of that argument is; to me it just says that if things were different, then things would be different and we wouldn't be here to observe them.
One possibility that has a certain appeal to me is the notion that there's an infinity of possible values for those constants, and thus an infinity of possible universes, very few of which would be capable of sustaining life. We just happen to live in one of the ones that does, though it's worth noting that most of the universe we can see is pretty hostile to our kind of life.
Another possibility that also appeals to me is the idea that the values of those constants we measure may be the only possible ones, for deep reasons we don't understand yet. They may emerge as the smarter folks delve deeper into things like string theory and loop quantum gravity, but right now the best we can say is, "we don't know."
One thing is sure though. The two most successful physical theories we have, general relativity and quantum mechanics, despite being confirmed to umpteen decimal places repeatedly by every test anyone's ever been able to devise, remain fundamentally incompatible. There's at least one more layer of reality we haven't discovered yet, which might be strings, or quantum loops, or something else nobody's thought of yet. But invoking a designer at this point is the end of the research program; in one sense it explains everything, but in another way it explains nothing, it merely denies the need for further explanations.
And if you really want a headache, study up on Godel's Incompleteness Theorem: scientific proof that not only don't we know everything, we can't, not even in principle.