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The Big Bang is the leading explanation about how the universe began. At its simplest, it says the universe as we know it started with a small singularity, then inflated over the next 13.8 billion years to the cosmos that we know today.

Dark Matter is a form of matter thought to account for approximately 85% of the matter in the universe. It is not directly observable because it does not emit light or energy, but its presence is inferred from gravitational effects on visible matter, radiation, and the structure of the universe.

Redshift occurs when the visible light from an object is shifted towards the red end of the spectrum. This is indicative of an object moving away from the observer, and in cosmology, it is evidence for the expansion of the universe.

The Cosmic Microwave Background is the thermal radiation left over from the time of recombination in Big Bang cosmology. Discovered in 1965, it provides a snapshot of the infant universe when it was just 380,000 years old.

Hubble's Law is the observation in physical cosmology that galaxies are moving away from each other at speeds proportional to their distance. In formula terms, it's expressed as $v = H_0 imes d$, where $v$ is the galaxy's velocity, $H_0$ is the Hubble constant, and $d$ is the distance to the galaxy.

The Cosmological Principle is the notion that the distribution of matter in the universe is homogeneous and isotropic when viewed on a large enough scale, meaning it looks the same in all directions and from all points of view.

Cosmological Redshift is the redshift caused by the expansion of the universe. Light from distant galaxies is stretched, or redshifted, as the universe expands, with the degree of redshift being used to determine the rate of expansion.

Dark Energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe. It is the most accepted hypothesis to explain the observations since the 1990s that the universe is expanding at an accelerating rate.

The Inflationary Universe is a theory in physical cosmology which proposes that following the Big Bang, the universe underwent a period of rapid expansion faster than the speed of light. This expansion ironed out any irregularities and made the observable universe appear flat and uniform on large scales.

A quasar is a highly luminous active galactic nucleus in which a supermassive black hole with mass ranging from millions to billions of solar masses is surrounded by a gaseous accretion disk. Quasars are extremely distant objects and some of the oldest observable entities in the universe.

The Multiverse is a hypothetical group of multiple universes. Together, these universes comprise everything that exists: the entirety of space, time, matter, energy, and the physical laws and constants that describe them. The different theories of the multiverse vary in the nature of these universes.

The Anthropic Principle is the philosophical consideration that observations of the physical Universe must be compatible with the conscious life that observes it. This principle is often used in cosmology to explain the physical properties of the universe that are surprisingly suitable for life.

The Observable Universe is a term used in cosmology to refer to the portion of the universe that can be seen from Earth or its space-based telescopes and exploratory probes, due to the limitation imposed by the finite speed of light.

The Cosmic Event Horizon is a boundary in space-time beyond which events cannot affect an observer on Earth; events occurring beyond this horizon are not observable. The presence of a horizon is due to the expansion of the Universe, and its distance changes over time.