Or we should say, "the Theory of Universal Expansion"
Edwin Hubble was one of the first to classify galaxies other than our own. But he himself classified only a handful compared to how many we know that there are now. The telescope named after him, which orbits our Earth to avoid the distortion of peering through our atmosphere discovered thousands upon thousands of galaxies in a tiny patch of sky that astronomers previously thought was empty. We've had to up our estimate of how many galaxies there are in the Universe several times since.
When writing about the Universe it's important to keep this sense of scale in mind. It is frustrating to mark work of students who clearly understand and have spent a lot of time memorising facts and explanations only for them not to get the mark because they talk about when the world began with a huge explosion that we call the big bang. There are hundreds of billions of stars in our galaxy. An estimate which keeps getting increased. Many of those stars having systems of planets, of worlds, around them. And there are billions and billions of galaxies in our universe, and it is those galaxies that Hubble studied, and those galaxies that he found were all receding from one another.
So Hubble used a special type of variable star, that he could find within distant galaxies to measure the distance from Earth to them. And he used a hundred year old effect discovered by Christian Doppler to measure the speed that those galaxies were moving away from Earth. He found that the wavelength of the light from those galaxies was shifted towards the red end of the spectrum. In other words the light waves had a longer wavelength than was expected, a phenomenon we call red shift.
By looking at how red-shifted the galaxies were he could calculate their speed, and so plot their distance from Earth against this speed. He found that most important relationship in Physics, proportionality! It took a lot of peer review and his getting a much larger set of data for his evidence to be accepted in the scientific community, for what it was; very strong evidence that the Universe is expanding.
This section of Physics is often used in GCSE as an example of how theories get accepted or rejected. There was a great deal of discussion following the publication of this new set of evidence and two rival theories became prominent during the twentieth century. George Lemaitre used Hubble's data to suggest that the Universe began from a single point, i.e. all the mass and energy in the Universe once occupied the same piece of space and time, and it has expanded ever since. This was nicknamed rather scathingly "the Big Bang Theory". The second theory, the main exponent of which was Fred Hoyle, stated that the Universe has the same overall density, and always has had, but that as it expands, new matter is created in the spaces left by galaxies receding from one another. This theory contained the cosmological principle, an idea that was accepted from Sir Issac Newton's time, that all observers everywhere would see an observable Universe with much the same physical properties. Ideas of Newton's always were difficult to shake, even in the light of new evidence.
The problem was that both theories, accounted successfully for Hubble’s Law, this proportionality between recessional velocity and distance from Earth. It would take some other piece of evidence to eventually decide between the two camps of astronomers. Like a lot of great science it was discovered when two scientists, Penzias and Wilson, were looking for something else. Or rather were trying to clear some frustrating background radiation which was stopping them getting a clear radio signal back from some NASA satellites. They checked their equipment thoroughly, swapping out parts, expecting to find some faulty piece which would eventually yield them a "noise free" signal, they found nothing. They even thought that it might have been a nesting pigeons, relieving themselves in their radio telescope array. They cleaned it, but the mysterious radio signal that they found everywhere they pointed their telescope was still there. They concluded it was there and they published their findings not knowing the significance.
Their paper was read and seized upon by exponents of the big bang theory as the evidence they needed to prevail in this debate. They called this radiation Cosmic Microwave Background Radiation. The same radiation, coming from every point in the universe; the same radiation, emitted during the initial rapid expansion at the dawn of time and space as high energy gamma radiation, which has been red-shifting for 13.8 billion years and is now at the exact same frequency everywhere, in the microwave portion of the electromagnetic spectrum. Radiation that they styled as "the smoking gun", the final evidence to reject the steady state theory and accept the big bang theory. Subsequent space missions have confirmed this same radiation and have lead to the theory being the dominant explanation for the origin of our universe.
Scientists are still refining the Hubble constant, which is the gradient of the proportional graph has inverse time units. In other words it is inverse the age of the Universe. The most up-to-date estimate of this is 13.8 billion years old, which is… not that old really. It's an amazing piece of science, looking for the available data and extrapolating back to an origin, literally the origin of a graph. There is loads more debate to be had though, with now our theories of the eventual fate of the Universe being the hot debate, will we expand forever, or collapse back to another tiny single point, or will this weird dark energy stuff mean that we're expanding at an accelerating rate.
At any rate it's a fascinating area of study in your GCSE, again focus on the sequence, but look carefully at the objectives, and get straight what the examiners want you to get from it!
The last bit I like to talk about is religion. This is something that is discussed in the Religious Studies GCSE. Does all this mean that there is no room for faith, no room for a creator? We will never know what was before the big bang, that evidence is lost to science forever. There is no experiment that we can ever do which will gather evidence for anything before that event. So believe if you will, or not, but don't let Physics, and adapting your understanding of the Universe based on evidence get in the way of your faith. After all, Lemaitre, the original author of the big bang theory was himself a Roman Catholic Priest.