In the 1920s, Edwin Hubble conducted groundbreaking research correlating galaxy distance with recession speed, a pivotal discovery that laid the foundation for understanding an expanding universe. Hubble's observations revealed a pattern: galaxies closer to ours recede at a slower pace, while those farther away move away more rapidly. This relationship, known as Hubble's Law, challenged the notion of cosmic centrality and highlighted an evolving cosmos.

To explain this cosmic expansion, a two-dimensional analogy was introduced, envisioning the universe as the surface of a balloon. In this model, the Big Bang, the event marking the universe's origin, is likened to the expansion of the balloon from a central point. This concept illustrates that the universe is larger in the future and was smaller in the past. Georges Lemaitre's theoretical work, compatible with Albert Einstein's General Theory of Relativity, supported the idea that the universe originated from a "primaeval atom."

Delving into questions about the universe's infancy, the analogy suggests that looking far into space equates to peering back in time. The deeper we gaze into the cosmos, the younger the universe we observe. With advancements in telescope technology, there is potential to witness pivotal moments, like the birth of galaxies, dating back billions of years.

However, profound mysteries persist, such as the elusive time-zero and the conditions preceding it. According to the theory of relativity, the density at time-zero was infinite, presenting a challenge that current physics, lacking a quantum theory of gravitation, is yet to address. The exploration of this phase in the universe's history remains one of the most intriguing and unsolved puzzles in contemporary physics.

Inflation, dark matter and dark energy,

Despite early scepticism, the Big Bang theory gained support in 1982 with Alan Guth's proposal of the Inflationary Big Bang theory, suggesting an extraordinary expansion when the universe was incredibly young. This theory, resolving previous issues, was later substantiated by evidence such as fluctuations in cosmic background radiation.

The Inflationary Big Bang theory posits that quantum fluctuations during inflation generated the universe's seeds. Testing this theory, the 1992 Cobe satellite confirmed predicted fluctuations, earning George Smoot and John Mather the 2006 Nobel Prize in Physics.

Inflation likely resulted from a phase transition, akin to water freezing to ice, releasing latent energy and causing rapid expansion during the Big Bang. Dark matter, a mysterious substance, constitutes a significant portion of the universe's mass, exceeding visible matter. Despite speculation, its true nature remains elusive.

The future expansion of the universe hinges on its mass content. A high mass could lead to a closed universe, eventually contracting, while a low mass suggests an open universe, expanding indefinitely. Discoveries in 1998 revealed an unexpected acceleration in the universe's expansion, attributed to "dark energy," constituting 74% of the universe's mass-energy.

Recent measurements suggest the universe comprises 4% normal matter, 22% dark matter, and 74% dark energy. This revelation underscores our limited understanding, as we only comprehend 4% of the universe's composition, leaving 96% shrouded in mystery