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The prevailing cosmological model explaining the origin of the universe, suggesting it began as a singularity around 13.8 billion years ago, and has been expanding ever since.

A hypothetical form of energy that permeates all of space and accounts for the accelerating expansion of the universe.

The increase in wavelength (and corresponding decrease in frequency) of light from an object moving away from the observer, used as evidence for the expanding universe.

The assumption that the universe is homogeneous and isotropic on large scales, which is fundamental to cosmological models.

A theory proposing a period of extremely rapid expansion of the universe shortly after the Big Bang, solving the horizon and flatness problems.

A type of matter that does not emit electromagnetic radiation, making it invisible and detectable only through its gravitational effects.

The observation that the recessional velocity of galaxies is proportional to their distance from us, which supports the theory of an expanding universe.

The rate at which the universe is expanding, given in kilometers per second per megaparsec (km/s/Mpc).

Ordinary matter consisting of protons, neutrons, and electrons, which makes up the visible component of the universe such as stars and planets.

The theoretical study and modeling of how small primordial inhomogeneities evolved into the stars, galaxies, and clusters observed in the universe today.

The redshift of distant galaxies caused by the expansion of the universe, described by the scale factor relating the observed wavelength to the emitted wavelength at the source.

The paradox that a static, infinite, and eternal universe would result in a bright night sky, while in reality the night sky is dark, hinting at a dynamic and finite universe.

The standard model of cosmology that includes the cosmological constant ($\Lambda$) associated with dark energy, and cold dark matter (CDM), fitting the large-scale structure and observations of the universe.

The thermal radiation left over from the time of recombination after the Big Bang, and is a vital source of information on the early universe.

A philosophical consideration that observations of the universe must be compatible with the conscious life that observes it, affecting the interpretation of cosmological data.

Large-scale structures composed of gravitationally bound galaxies and clusters, forming a web-like pattern in the universe with vast empty voids in between.

The maximum distance from which light from objects could have traveled to the observer in the age of the universe, delineating the observable part of the universe.

The bending of light by massive objects, such as galaxies or clusters of galaxies, illustrating Einstein's theory of general relativity and helping to map dark matter.

The epoch in the early universe when free electrons combined with protons to form neutral hydrogen, leading to the decoupling of matter and radiation.