Imagine rewinding the universe, going back in time through billions of years until all the stars, galaxies, and planets vanish. What would remain? According to one of the most widely accepted explanations in cosmology, called the Big Bang Theory, the entire universe began as an incredibly small, hot, and dense point, often described as a "singularity." Over the course of billions of years, it expanded and cooled, evolving into the vast cosmos we see today.

The Big Bang Theory does not describe an explosion in space, but rather an expansion of space itself. Approximately 13.8 billion years ago, everything—matter, energy, space, and time—was compressed into something unimaginably tiny. In a fraction of a second, the universe began to expand. As it expanded, it cooled, allowing particles to form. Eventually, these particles combined to create atoms. Over millions of years, these atoms clumped together under gravity to form stars, galaxies, and all the structures we observe in the cosmos.

One of the major pieces of evidence supporting the Big Bang Theory is the cosmic microwave background (CMB). In the 1960s, scientists discovered a faint glow of microwave radiation coming from every direction in space. This glow is like a fossil of the early universe, a remnant of the heat from when the universe was extremely hot and dense. The CMB is astonishing because it is almost the same temperature everywhere, which suggests that the universe began as a hot, uniform state before expanding and cooling over time.

Another important clue comes from observing how galaxies move. Scientists have found that most galaxies are moving away from us, and the farther away a galaxy is, the faster it seems to be receding. This observation, made famous by astronomer Edwin Hubble, suggests that the space between galaxies is stretching, pushing them apart—evidence that the universe is still expanding. If we imagine turning this process in reverse, we find that galaxies would have been much closer together in the past, reinforcing the idea that the universe began in a more compact form.

The Big Bang Theory also predicts the amounts of different elements created in the early universe. Right after the big bang, the universe was mostly hydrogen and helium, with tiny amounts of other light elements. When scientists measure the composition of old stars, they find that these predicted proportions are remarkably accurate. This agreement between theory and observation gives astronomers confidence that the Big Bang Theory is on the right track.

Despite its success, the Big Bang Theory does not answer every question. For instance, it does not explain why the initial singularity existed in the first place. It also doesn't fully account for phenomena like dark matter and dark energy, mysterious components that make up most of the universe's mass-energy content. Scientists continue to search for more clues, using powerful telescopes and particle accelerators to test new hypotheses and refine existing theories.

In recent years, astronomers have discovered that the expansion of the universe is actually speeding up, not slowing down. This unexpected finding led to the idea of dark energy, a kind of "antigravity" force pushing galaxies apart faster and faster. While this does not contradict the Big Bang Theory, it shows that our understanding of the universe’s past, present, and future is still evolving.

Nevertheless, the Big Bang Theory remains our best explanation for how the universe began and evolved. It is supported by multiple lines of evidence—the cosmic microwave background, the expansion of the universe, and the observed mixture of elements. By studying the universe’s earliest moments, scientists hope to understand not only where we came from but also where we might be going. The Big Bang Theory teaches us that the cosmos is dynamic, changing over time, and that through observation, imagination, and scientific reasoning, we can uncover the secrets of existence.