Re: Relativity - Great! But what about String Theory, Dark Matter and Dark Energy?
"But it is ONLY hypothesised to exist as an explanation for these observations"
Well yes, that is how hypotheses work. Would you prefer it was hypothesised to exist for observations we don't actually have, or for ones which it wouldn't explain? That wouldn't make an awful lot of sense. Science really isn't all that complicated. In essence:
1) Make some observations;
2) Come up with a hypothesis that can explain them;
3) Figure out what other consequences said hypothesis might have;
4) Continue making more observations and checking if they match what's expected.
That's it. See something, try to explain it, see if that explanation works, GOTO 10. I've never understood why anti-science types constantly complain about things like a hypothesis only being invented to explain some observations we've made. That's the whole bloody point, and is the only vaguely sane way to approach things.
"What is dark matter? The stuff which explains the rate of expansion of the universe. What explains the rate of expansion of the universe? Dark matter."
Dark matter has nothing to do with the expansion of the universe*. Dark matter stems from observations that there often appears to be more mass present in some places than we can actually see. Galactic rotation curves are probably the best known observation - stars towards the outer edges of galaxies are almost always seen to be orbiting faster than they would if the stars and gas in the galaxy were the only mass in it. Dark matter isn't some bizarre invention out of thin air, it's just by far the simplest explanation - there appears to be more mass than we can see, therefore there's probably more mass that we can't see. Note that this doesn't even need to get the weirdness of relativity and quantum physics involved; orbits can be largely explained by Newtonian mechanics and observations made using Newtonian telescopes.
It only starts getting complicated because obviously the first thing everyone thought of was that it was just regular matter that we couldn't see in the form of things like brown dwarves and diffuse gas clouds, but after looking into it it turns out that can't actually explain things (essentially, if it was just lots of little things too faint to see, there would have to be so much that we'd be able to see it). And then as time passed a whole bunch of other, entirely separate, observations were made that not only suggested there was more mass around than we can see, but also tended to agree on how much, where it is, and that ordinary baryonic matter can't be present in the quantities required.
Which is why we now have more seemingly weird theories about what it could be. No matter what we look at or how we look, we consistently see indications that there's much more mass out in space than we can see directly via the electromagnetic spectrum. It can't be made out of baryonic matter, therefore it must be made of something else. We've ruled out lots of something elses so far, but we're still not sure what it actually is. Hence, dark matter. You certainly could call it sky pixies instead if you liked, but that wouldn't change the observations or the best answers we've come up with to explain them.
* OK, that's not quite true. One of the many lines of evidence pointing at dark matter is the observation that the rate of expansion of the universe is consistent with there being a lot more matter around than we can see. Dark energy is an entirely separate theory related to acceleration of the rate of expansion, but dark matter is required as well to actually fit the observations.