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Dark matter is the matter which fills galaxy with mass which is required to even maintain their shape. It
does not interact with fermions and bosons with make up the ordinary matter. As light particle(photon) is
a part of boson ,it is not visible. Due to its gravitational pull It also slows the acceleration of
the universe.

Composition of the matter in the universe: –

• Approximately 5% is the matter that we can see, touch and all the stars and galaxies are
made of. This matter is made up of fermions which interacts with bosons which makes all the
events.
• Approximately 27% of the matter is made of dark matter.
• And the rest 68% is made up of dark energy which is a repulsive force which causes the
expansion of the universe.

How was the dark matter hypothesis proposed: –

• A Swiss American astronomer named Fritz Zwicky inferred the existence of dark
matter in 1933.
• The black hole in the middle of galaxies constituted mass less than 1% of the galaxies
mass which led to the question that how were these galaxies even existed as
everything would tore apart due to acceleration of the universe.
• It was noted that all the visible mass in the galaxy only constituted 1 % of the mass
required.
• So it is hypothesised that due to the gravitational pull of the dark matter everything
in the galaxy stays together.

More observations of dark matter:-

• Galactic Rotation Curves : Astronomers observed that the outer regions of galaxies rotate at
higher speeds than calculated using visible mass alone.The gravitational pull is insufficient to
explain this high speed which suggests that it is caused due to the gravitational pull of dark
matter.
• Gravitational Lensing: Einstein’s general theory of relativity prosposed gravitational lensing
or the light bending effect which became an indicator to the unseen mass or dark matter.
According to Einstein’s theory, light bends around gravitational fields. The amount of lensing
or bending in galaxy cluster suggests the presence of additional mass (dark matter) which is
not emitting light.
• Cosmic microwave Background (CMB0: The study of CMB which is the remnant radiation of
the early universe or big bang provides information on the distribution of matter. Dark
matter is thought to have influenced the large-scale structure observed in CMB radiation.
• Large-Scale structure: The large-scale distribution of galaxies ad galaxy structures in the
universe is influenced by the gravitational pull of the dark matter. Simulations that include
dark matter help reproduce the observed cosmic web of structure.

Theories about and formation of dark matter:-

• Primordial black holes: In the 70s after inferring the existence of dark matter, Stephen
Hawking proposed idea that dark is made up of primordial black holes which are very small
in size and were created moments after the creation of the universe or big bang.
• Weak interacting Massive Particles(WIMPs): This theory suggests that dark matter is made
up of a particles which interact less with ordinary matter (particles that were proposed by
quantum mechanics).
• Axions: Axions are hypothetical elementary with low mass and are another candidate of dark
matter. They were originally introduced to solve certain problems in particle physics but are
also considered a potential constituents of dark matter.