Fritz Zwicky discovered a discrepancy between visible matter in clusters of galaxies and their motions, suggesting the presence of invisible matter, or "dark matter," interacting gravitationally with the visible matter. This led to the discovery of dark matter, which now accounts for about 27% of the Universe. Modern calculations suggest dark matter makes up about 68% of the Universe, leaving only 5% visible to us. The expansion of the Universe is accelerating, a surprising discovery due to the gravitational attraction between galaxies and clusters. Dark energy, the unseen repellant force, makes up about 68% of the Universe.
Dark matter is a mysterious substance that exists in the universe, but its properties are not visible. Gravitational lensing measures the gravitational pull galaxies exert on light, providing insight into dark matter's distribution and amount. The best explanation for dark matter is the existence of Weakly Interacting Massive Particles (WIMPs), but direct observation of these particles has been elusive. Einstein's Theory of General Relativity predicted the Universe was not static, leading to the addition of a "cosmological constant" to counter gravity. The cosmological constant represents the energy of empty space, which is constant throughout space and time.
Other interpretations include dark energy being an energy field that varies over time and space, or that gravity may not be fully understood. Astronomers are currently testing modifications to General Relativity to explain the Universe's accelerating expansion.