As Endeavour launches for the last time, it will be carrying an absurdly expensive particle detector along with it. The Alpha Magnetic Spectrometer will mount on the ISS and search space for antimatter, dark matter, dark energy, and even stranger things, like strangelets.
The first Alpha Magnetic Spectrometer made it to space on Discovery back in 1998, and it worked well enough that $33 million was set aside to build a bigger, more sensitive version. About $2 billion later, construction was completed, although the new, bigger, better Alpha Magnetic Spectrometer doesn't actually incorporate the fancy cryogenic magnets that were supposed to make it work properly in the first place, and there are some questions as to whether or not it's going to be capable of doing everything that it's promised to do.
In any case, it's now (or, will soon be) headed to space on Endeavour, where it will plug into the ISS and get to work. The instrument has several science objectives. For starters, it'll be looking for antimatter, specifically antihelium, since there's supposed to be lots of antimatter around (left over from the Big Bang) but we have no idea where it's all hiding. Also hiding somewhere is dark matter and dark energy, which together make up something like 95% of the universe by mass, but we've never seen any of it.
If we get lucky, the AMS will also find some evidence of strangelets. All matter on Earth is made of quarks, and even though there are six different types of quarks (up, down, charm, strange, top, and bottom), we've only seen the up and down variety in matter. It's possible, though, that some strange quarks could be tossed into the mix, which would make an entirely new type of matter called a strangelet.
The other thing that AMS will be good for is taking measurements of what the space environment is like. If we're ever going to make it to Mars without turning into mutant space-beasts, it would be good to know how much radiation shielding is going to be necessary, and the AMS will be operational long enough to figure that out. Originally slated to return to Earth via shuttle, the AMS will be collecting data for at least three years, and if all goes well, it'll keep running for a decade or more.