Cosmology is the story of the universe. It’s a science that asks the biggest questions possible: Where did everything come from? What is it made of? And where is it all going? It tackles the entire cosmos, from its first moments to its ultimate fate.

The prevailing scientific theory for how the universe began is the Big Bang. This wasn't an explosion in space, like a firework. It was the beginning of space itself. About 13.8 billion years ago, the entire universe was squeezed into an incredibly hot, dense point. From that single point, everything expanded outwards.

In the first fractions of a second, the universe was a chaotic soup of fundamental particles and energy. As it expanded, it cooled down. This cooling allowed particles to combine, eventually forming the first atoms—mostly hydrogen and helium. Every star, planet, and galaxy we see today is made from the stuff forged in that initial event and its aftermath.

How do we know the universe is expanding? In the 1920s, astronomer Edwin Hubble observed that nearly all galaxies are moving away from us. More importantly, he found that the farther away a galaxy is, the faster it’s moving away.

This is a telltale sign of a universe that is stretching. Think of a loaf of raisin bread dough rising in an oven. As the dough expands, every raisin moves away from every other raisin. If you were sitting on one raisin, you'd see all the others moving away from you, and the ones farthest away would appear to move the fastest. This is happening with galaxies in our universe. Space itself is expanding, carrying the galaxies along with it.

This observation means if we run the clock backward, all the galaxies would rush back together toward a single point. This is the core evidence for the Big Bang—that the universe started from a much smaller, denser state.

If the universe was born in a hot, dense state, there should be some leftover heat. In 1965, two astronomers accidentally discovered it. They found a faint, uniform glow of radiation coming from every direction in the sky. This glow is called the Cosmic Microwave Background, or CMB.

The CMB is the afterglow of the Big Bang. For the first 380,000 years of its existence, the universe was so hot that it was an opaque, glowing fog. As it expanded and cooled, atoms formed, and the universe suddenly became transparent. The light from that moment has been traveling through space ever since.

This ancient light is the oldest thing we can see. The tiny variations in its temperature, shown as different colors on the map, were the seeds from which all the stars and galaxies eventually grew. The CMB provides a snapshot of the infant universe, and its properties match the predictions of the Big Bang theory perfectly.

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Together, the Big Bang theory, the expansion of the universe, and the Cosmic Microwave Background form the foundation of modern cosmology, giving us a powerful framework for understanding our cosmic origins.