The leading theory of the universe's origin is the Big Bang, which holds that the universe began about 13.8 billion years ago from a hot, dense state and has been expanding ever since. Strong evidence came in 1964 when Arno Penzias and Robert Wilson discovered the cosmic microwave background, or CMB, residual radiation from that early era, now observed at a temperature of about 2.725 K. About 380,000 years after the Big Bang, an era called recombination, electrons combined with nuclei and the universe became transparent, releasing the photons we see today as the CMB. Before the first stars ignited, the universe passed through its "dark ages," which ended when ultraviolet light from the earliest stars and quasars reionized the neutral hydrogen in an epoch known as reionization.
That the universe is expanding was demonstrated in 1929 by Edwin Hubble, whose observations showed that more distant galaxies recede faster, a relationship known as Hubble's law: \[ v = H_0 d \] The Hubble constant, \(H_0\), measures the current rate of expansion at about 70 km/s/Mpc. The motion of receding objects stretches their light to longer wavelengths, an effect called redshift, while approaching objects show blueshift. A period of extraordinarily rapid expansion, called inflation, is thought to have smoothed out the early universe, and on the largest scales the cosmological principle holds that the universe is homogeneous and isotropic, with observations indicating a flat overall geometry.
A surprising discovery was that the expansion of the universe is accelerating, driven by a mysterious component called dark energy that makes up about 68% of the cosmos. The cosmological constant, originally introduced by Einstein and later famously called his "greatest blunder," represents the energy density of empty space and remains a leading candidate for dark energy. The remaining matter budget includes ordinary (baryonic) matter and dark matter, an invisible substance making up about 27% of the universe that reveals itself through galactic rotation curves, gravitational lensing, CMB anisotropies, and the growth of cosmic structure. Leading dark matter candidates include WIMPs (weakly interacting massive particles) and axions, and a competing idea, MOND (Modified Newtonian Dynamics), instead proposes changes to the laws of gravity. The distinction between cold and hot dark matter also matters: cold dark matter, moving slowly compared to the speed of light, fits observed structure formation better than a hot alternative.
Some cosmological ideas venture further. The multiverse hypothesis suggests our universe may be one of many, while the anthropic principle observes that the universe's physical constants must be compatible with the existence of observers like us. The once-popular steady-state theory, which proposed a largely unchanging universe, was largely abandoned after the discovery of the cosmic microwave background, which fits naturally within the Big Bang picture. Together, these ideas frame cosmology's biggest questions about the origin, composition, and ultimate fate of everything we can see.