Echoes of the Big Bang
There are many things that we can observe about the universe which lead us to assert that it all started with a Big Bang. There is the velocity of galaxies further away from us being faster, or the fact that when we look back to a younger universe we see more primitive structures that would precede what our universe looks like now. But probably the best evidence of the Big Bang is called the Cosmic Microwave Background Radiation. Which sounds really cool, and totally is. So what exactly is it?
Some things you may or may not already know: light is a wave, and there's a whole EM spectrum, from very, very short wavelengths (gamma radiation) to much longer ones (radio waves). The visible spectrum sits in between those two, surrounded on the shorter wavelength by ultraviolet and on the longer side by infared. On the longer wavelength side, in the radio spectrum, there is a category of light wavelengths called microwaves. You may be familiar with them, as a microwave oven uses them to heat your food by oscillating water molecules in it. As they are in the radio frequencies, they're invisible to the human eye, but can be picked up by equipment designed to do so—like a radio.
Now, a very long time ago, just after the Big Bang, the universe was far too hot and energetic and compact to allow atoms to form. This meant that electrons and protons were all whizzing about in a giant sub-atomic soup, and light was constantly being scattered by this electromagnetic interference. At this point, the universe was essentially opaque to light. That's a weird thought, because it's nothing like anything we can relate to in our current time and age of the universe, because in our universe everything is cool enough that atoms exist, and electrons and protons aren't found outside atoms very often. So, at some point, the universe had to cool down, atoms would form, and then light would be able to travel unimpeded throughout the universe, like what we see today. This process happened about 300,000 years after the Big Bang, and was called recombination, although the subatomic particles weren't actually combined before, so the re- part of it is a little odd. Nevertheless, at about 300,000 years about the Big Bang, the universe finally became transparent to light, and that light has been streaming through the cosmos ever since.
Now, if that light is out there, we should be able to find it. This may seem a little counter-intuitive, as the first impression one would have is that the light would have passed us and be gone. But really, it's constantly hitting us. This is because the Big Bang didn't happen in a point in the universe, the Big Bang created the universe, and therefore the Big Bang happened at every point in the universe. Space itself is what's expanding, so every single place in the universe experienced the Big Bang equally long ago, and was equally the "center" of the Big Bang, as it were. This is really weird to think about, and I'll probably do another post getting into more detail on it at some point, but for now, what that means is that the light from recombination is still traveling toward us from places distant enough in the universe. Because looking at distant places in space is equivalent to looking back in time (due to the speed of light being finite), we could look back far enough to see the light from that 300,000 years after the Big Bang. And we can.
Because space is constantly expanding, light that has been traveling for a long time gets its wavelength expanded with space. One of the early pioneers of the Big Bang model, Ralph Alpher, predicted that this would put the light from the early universe in the microwave spectrum, and further predicted that we would be able to spot this "echo" of the Big Bang if we could develop sensitive enough equipment. At the time, the prediction was mostly ignored because there was no equipment that could do that kind of detecting, and a lot of astronomers were fans of a "steady state" model of the universe, that had neither a beginning nor an end. Much later, and when the Big Bang was starting to gain slightly more credulity and following, two astronomers working at Bell Labs, Penzias and Wilson, stumbled across some microwave spectrum radiation that they could not explain coming from space, while working with the most sensitive radio telescope at the time. Neither was familiar with the prediction of the CMB (Cosmic Microwave Background), and only through chance complaining about the microwave "noise" that they were unable to remove to another astronomer did they realize they had stumbled upon the find of the century. Evidence of the Big Bang itself, echoing through the ages. The radiation is faint, but anyone can still pick it up—turn on your radio, and tune it to be in between channels. Of that static you hear, a tiny, tiny part of it is caused by the radio picking up microwaves that have traveled though space for billions of years from when the universe was so young atoms had only just formed. And if that doesn't deserve the label "really cool," I don't know what does.