The space between galaxies, often called intergalactic space, is popularly imagined as a perfect vacuum, but this is far from accurate. While incredibly sparse compared to the space within a galaxy, this vast volume is filled with matter and energy that dictates the structure and evolution of the entire universe. Intergalactic space (IGS) is the region existing between galaxies, whether clustered together or separated by immense cosmic voids. Its contents, including diffuse gas, invisible matter, and pervasive fields, serve as the scaffolding for the largest structures in the cosmos.
The Intergalactic Medium: Diffuse Gas and Plasma
The most common form of ordinary matter between galaxies is the Intergalactic Medium (IGM), a highly diffuse gas consisting primarily of hydrogen and helium plasma. This baryonic matter is concentrated along the filaments of the cosmic web, the largest known structure in the universe. Even in these filaments, the IGM density is extraordinarily low, averaging only a few atoms per cubic meter, making direct detection difficult.
The IGM is heated by the gravitational collapse of matter into large-scale structures and by shockwaves from active galactic nuclei and merging galaxies. A significant portion of this gas is the Warm-Hot Intergalactic Medium (WHIM), a plasma with temperatures ranging from \(10^5\) to \(10^7\) Kelvin. Cosmologists estimate that the WHIM accounts for 40 to 50 percent of the universe’s total ordinary matter, solving the problem of the “missing baryons.”
The extreme heat of the WHIM causes its atoms to become highly ionized, stripping electrons from hydrogen and helium nuclei. This state makes the gas nearly invisible to optical telescopes since it does not emit light traditionally. Detection relies on observing its faint absorption or emission of low-energy X-rays and ultraviolet radiation. Researchers also use the thermal Sunyaev-Zel’dovich effect, which measures the scattering of cosmic microwave background photons by the hot gas, to confirm these tenuous filaments.
The Dominant Invisible Structure: Dark Matter
While the IGM is the ordinary matter in intergalactic space, the majority of the mass is Dark Matter (DM), an invisible substance constituting approximately 85% of the total mass in the universe. Dark matter does not interact with light, making it detectable only through its gravitational effects on visible matter. This invisible mass dominates the gravitational landscape of intergalactic space.
Dark matter acts as the structural foundation for the large-scale distribution of all other cosmic components, including the IGM and galaxies. Its gravitational pull dictated the formation of the cosmic web, featuring dense knots, extended filaments, and vast voids. The IGM is drawn into and contained by this dark matter scaffolding.
Even the emptiest regions, known as cosmic voids, are permeated by low densities of dark matter. Although voids have the lowest overall matter density, dark matter is still present at about 10 to 20 percent of the universal average. Computer simulations predict that even within these voids, dark matter forms miniature, faint tendrils that echo the larger cosmic web structure.
Stray Objects and Pervasive Fields
Beyond the bulk components of gas and dark matter, intergalactic space contains numerous isolated objects and fields. These include stars ejected from their host galaxies due to gravitational interactions, known as rogue stars. These solitary stars drift through the vast darkness, having escaped their galactic homes.
Intergalactic space is also filled with high-energy particles called cosmic rays, which travel at nearly the speed of light. These particles, primarily protons and atomic nuclei, originate from powerful astrophysical events like supernovae and active galactic nuclei. Their paths are influenced by the weak but measurable Intergalactic Magnetic Fields (IMFs) that thread the space between galaxies. The strength of the IMFs is hypothesized to be in the nanogauss range, strong enough to potentially deflect ultra-high-energy cosmic rays over immense distances.
The most pervasive energy component is the Cosmic Microwave Background (CMB) radiation, the thermal afterglow of the Big Bang. The CMB uniformly fills all of intergalactic space, representing the oldest light in the universe. This background radiation has cooled over billions of years to a temperature of about 2.7 Kelvin, providing a constant, low-energy bath of photons.