What's (the) Matter?
Matter. It's basically everything. Anything you can see and touch (except holograms), and some things that you can't see or touch, is matter. There's plain old boring matter, which is all the things you see around you all the time, and is composed of atoms you see on the periodic table. But then there's all that cool weird matter (not necessarily strange or exotic matter, because those are actual, real names of types of matter). Since you're already probably pretty familiar with prosaic matter, having been interacting with it all your life, let's talk about some of the more interesting matter.
Strange matter. To understand what this stuff is, you need to know what atoms are made of. Basically, each particle in an atom (eletrons, protons, and neutrons) is made up of even tinier particles called "quarks" (so much for the meaning of the term "atom," which comes from the Greek atomos, which means "indivisible"). These quarks have two qualities to them: color, and flavor. They can be red, green, or blue, and have a flavor of up, down, top, bottom, strange, or charm. Yes, those are real things. Most conventional matter exists as quarks with all three colors, and up and down quarks. In fact, all those partciles have to be made up of a "colorless" combination of quarks, meaning they need an even combination of red, green, and blue quarks (or they might have antired, antigreen, or antiblue to cancel out a color, but that's even weirder). Strange matter is simply matter which also consists of strange quarks, not just up and down. This strangeness is in more than just name. See, Strange quarks are named that way because they posess a physical quality called, aptly, "strangeness," which affects how they decay. Basically, they don't behave the way "normal matter" ought to. In fact, there's baryonic (your typical, everyday kind of matter, which must also be colorless, as I elaborated on previously) matter for each of the fun types of quarks, as there is "topness," "bottomness," and "charm" qualities, which all affect how matter behaves.
Exotic matter is more of an umbrella, under which everything else interesting falls. Basically, "exotic matter" is matter that is rare and weird, not really understood (but probably exists), is theoretical (and usually violated the laws of physics), or is in some other way atypical. The simplest of these types of matter is dark matter. Dark matter is basically matter that isn't. Or, to be more precise, it's matter that barely is. See, dark matter exists as matter, but only interacts with anything else via gravity. It doesn't reflect, or even absorb light (so it's not necessarily "dark," more like "invisible"). In fact, it doesn't interact with the EM spectrum in any way (not even through black body radiation). And ths matter is what gives our galaxy (and all other galaxies) the mass they have. In fact, its existence can only be inferred due to galactic rotation curves—the visible matter in the galaxy can't account for the way our galaxy rotates, so there has to be a so-called "dark matter halo" around our galaxy to keep it spinning the way it does. Or maybe it's just magic (it's not magic).
Another cool piece of exotic matter that is theorized to exist is a quark-gluon plasma. Basically, this stuff is a high-energy plasma or sub-atomic particles that doesn't exist in the form of discrete atoms (the quarks and the gluons roam like antelope on the range). Quarks we covered above, and gluons are "force carriers" for the stong force between quarks—the particle that carries the force that sticks quarks together (like glue. Hence, "gluons." Aren't physicists clever?). In this super-high energy state, quarks don't form any larger particles, and the only place it's theorized to exist is in the core of a neutron star—a black hole that didn't quite make the cut, and collapsed into a giant atomic nucleus that's usually highly magnetized and spinning (called a pulsar. Fun related fact: A highly magnetized spinning galaxy is called a quasar). So at crazy high energy levels, it's thought that quarks become asymptotically free. The only other place this kind of matter is theorized to exist was at the very beginning of the universe, just a few milliseconds after the Big Bang.
The other kinds of matter under this umbrella are ones that more or less violate the known laws of physics, but someone, somewhere, thinks they may exist. The most well-known of these would be tachyons—particles which travel backwards in time. They have never been observed toi exist, and violate the laws of causality, and many other laws, as the reason for a tachyon's direction of temporal travel has to do with its mass. See, anything with mass cannot exceed or even reach the speed of light (due to the time/velocity equations in relativity), but tachyons have imaginary mass, that is, mass that contains a mathematically imaginary component (some number multiplied by the square root of -1, written as i). Because of this, these particles would be mandated by those same equations to constantly be exceeding the speed of light. And, in doing so, they would travel backward in time (which can be shown with some really cool Lorenz transformation diagrams that I'm not going to get into in this blog post). Another kind of crazy matter with regard to mass is matter that has negative mass—which would do strange things, like move in the opposite direction of an applied force. Interestingly, negative mass doesn't conflict with any of the math involved in physics, but it violates all the ways matter is known to behave, and there is nothing in the observable universe that seems to support its existence. But still, it's not something that is impossible, per se.
The point of this incredibly meandering blog post is that there are all sorts of strange things, real, theoretical, or possibly imagined, out there in the universe. Even something as simple as the stuff we interact with every day at all times can be strange and exotic if you look close enough at it (though not always. Some matter is just boring, ya' know?). And even then, all this matter makes up less than 30% of the universe—and the matter that we can see and observe (basically, matter that isn't dark matter) makes up less than 5% of the universe. Everything in the universe that we can see is only 5% of what there is. Isn't that crazy? That's totally crazy. Crazy awesome.