The question of “how much does the universe weigh” is challenging because “weight” implies a gravitational force acting upon the universe from an external source. Scientists instead focus on the total mass-energy content—the sum of all matter and energy within a defined volume of space. This mass-energy dictates the geometry and ultimate fate of the cosmos, according to Albert Einstein’s theory of General Relativity. While the total extent of the universe is unknown, cosmologists have developed a robust estimate for the mass-energy content contained within the portion we can observe. This calculation relies on theoretical models combined with highly precise measurements of the universe’s fundamental properties.
Defining the Observable Universe
The first step in calculating the universe’s content is defining the boundaries of the measurement. The “observable universe” refers to the spherical region of space containing all the matter from which light or other signals have had time to reach Earth since the Big Bang. This limit is imposed by the finite speed of light and the age of the universe, approximately 13.8 billion years.
The expansion of space complicates the calculation of this region’s radius. As photons traveled toward us, the fabric of space stretched, moving the source of that light farther away. The objects that emitted the light we see today have since moved to a much greater distance.
The current distance to the most remote light sources is estimated to be about 46.5 billion light-years in every direction. Consequently, the observable universe is a sphere with a diameter of roughly 93 billion light-years. All calculations of the universal mass are confined strictly to the estimated content within this immense volume.
Calculating Cosmic Mass Density
Scientists determine the total mass-energy content by focusing on its average density, rather than attempting to count every star and galaxy. This approach is based on the cosmological principle, which holds that on the largest scales, matter and energy are distributed uniformly throughout space.
The foundation of this calculation is the “Critical Density.” This theoretical value represents the precise density required for the universe to have a flat geometry, meaning its overall shape is not curved. If the actual density were greater than the critical density, space would be closed and curve back on itself; if it were less, space would be open and curve outward.
Observational data from satellite missions, such as the Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck satellite, have been instrumental in determining the actual density. These instruments measured fluctuations in the Cosmic Microwave Background (CMB) radiation, the afterglow of the Big Bang. The pattern of these fluctuations provides a direct map of the universe’s geometry.
These measurements confirm that the universe’s actual density is extremely close to the calculated critical density. This critical value is estimated to be about \(0.85 \times 10^{-26}\) kilograms per cubic meter, equivalent to roughly five hydrogen atoms within every cubic meter of space. Knowing this average density and the total volume of the observable universe allows cosmologists to multiply the two figures. This yields the total mass-energy estimate, which is approximately \(10^{53}\) kilograms.
The Cosmic Inventory: Components of the Universe’s Mass-Energy
The density calculation reveals a surprising breakdown of the universe’s composition. The total mass-energy budget is dominated by components that are entirely invisible and mysterious, overshadowing the familiar matter that makes up stars and planets. This inventory is divided into three main categories, determined with high precision from cosmological models.
Normal or Baryonic Matter
This is the smallest portion, accounting for only about 4.9% of the total mass-energy. Normal matter is composed of protons and neutrons, which forms all the atoms that make up stars, galaxies, planets, gas clouds, and every visible object in the cosmos. All the brilliant stars, nebulae, and galaxies that astronomers have studied represent less than five percent of the universe’s total content.
Dark Matter
Dark Matter constitutes approximately 26.8% of the mass-energy budget. This substance is inferred to exist because of its powerful gravitational influence on visible matter. For instance, galaxies rotate much faster than the gravity from their visible stars alone could sustain, suggesting a large, unseen mass is holding them together. Dark matter does not absorb, reflect, or emit any electromagnetic radiation, meaning it is entirely invisible to telescopes across the entire spectrum. Its nature remains one of the greatest unsolved puzzles in physics, with leading theories suggesting it is composed of some as-yet-undiscovered subatomic particle.
Dark Energy
Dark Energy makes up the remaining and largest share at about 68.3% of the total mass-energy. Unlike dark matter, which pulls things together through gravity, dark energy is thought to be a form of energy inherent to space itself that exerts a repulsive pressure. Its existence was discovered when observations showed that the expansion of the universe is accelerating, rather than slowing down due to gravity.
Dark energy acts as a cosmic anti-gravity force, pushing galaxies further apart at an ever-increasing rate. The precise nature of dark energy is unknown, but its overwhelming proportion means that the ultimate fate and geometry of the cosmos are largely determined by this mysterious force. The combined total of dark matter and dark energy means that roughly 95% of the observable universe’s mass-energy content is completely unknown to us.