Dark matter is a theorized form of matter that is believed to account for approximately 80% of the matter in the universe,
and about a quarter of its total energy density.
The majority of dark matter is thought to be non-baryonic in nature,
possibly being composed of some as-yet undiscovered subatomic particles.
Dark matter has not been directly observed,
but its presence is implied in a variety of astrophysical observations,
including gravitational effects that cannot be explained unless more matter is present than can be seen.
For this reason,
most experts believe dark matter to be ubiquitous in the universe and to have had a strong influence on its structure and evolution.
The name dark matter refers to the fact that it does not appear to interact with observable electromagnetic radiation,
such as light,
and is thus invisible to the entire electromagnetic spectrum,
making it extremely difficult to detect using usual astronomical equipment.
The primary evidence for dark matter is that calculations show that many galaxies would fly apart instead of rotating,
or would not have formed or move as they do,
if they did not contain a large amount of unseen matter.
Other lines of evidence include observations in gravitational lensing,
from the cosmic microwave background,
from astronomical observations of the observable universe's current structure,
from the formation and evolution of galaxies,
from mass location during galactic collisions,
and from the motion of galaxies within galaxy clusters.
In the standard Lambda-CDM model of cosmology,
the total mass-energy of the universe contains 4.9% ordinary matter and energy,
26.8% dark matter and 68.3% of an unknown form of energy known as dark energy. Thus,
dark matter constitutes 84.5% of total mass,
while dark energy plus dark matter constitute 95.1% of total mass-energy content.
Because dark matter has not yet been observed directly,
it must barely interact with ordinary baryonic matter and radiation.
The primary candidate for dark matter is some new kind of elementary particle that has not yet been discovered, in particular,
weakly-interacting massive particles,
or gravitationally-interacting massive particles.
Many experiments to directly detect and study dark matter particles are being actively undertaken,
but none have yet succeeded.
Dark matter is classified as cold, warm,
or hot according to its velocity.
Current models favor a cold dark matter scenario,
in which structures emerge by gradual accumulation of particles.
Although the existence of dark matter is generally accepted by the scientific community, some astrophysicists,
intrigued by certain observations that do not fit the dark matter theory,
argue for various modifications of the standard laws of general relativity,
such as MOND, TeVeS,
or entropic gravity.
These models attempt to account for all observations without invoking supplemental non-baryonic matter. 
