📖 In-Depth Theory
Dark Matter
DARK MATTER is matter that does not emit, absorb or reflect electromagnetic radiation — it is invisible to all telescopes.
EVIDENCE FOR DARK MATTER:
GALAXY ROTATION CURVES:
Stars at the outer edges of galaxies orbit at the same speed as those near the centre.
Gravitational theory predicts outer stars should orbit slower (like planets in the solar system).
The extra gravitational force required suggests ~85% of matter in galaxies is invisible.
GRAVITATIONAL LENSING:
Massive objects bend light (gravitational lensing).
The amount of lensing observed is much greater than expected from visible matter alone.
The 'missing mass' is dark matter.
COSMIC MICROWAVE BACKGROUND:
Analysis of CMB fluctuations is consistent with dark matter making up ~27% of the universe.
WHAT IS IT?
Unknown — current candidates include: WIMPs (weakly interacting massive particles), axions, sterile neutrinos.
Has not been directly detected despite many experiments.
Dark Energy
DARK ENERGY is an unknown form of energy that is causing the expansion of the universe to ACCELERATE.
DISCOVERY:
In 1998, observations of distant supernovae showed the universe is expanding faster than expected.
The expansion should be slowing down (due to gravity between all matter).
Instead, it is SPEEDING UP.
EVIDENCE:
Type Ia supernovae as standard candles — known luminosity allows distance measurement.
Distant supernovae found further than expected — universe expanded more than predicted.
This requires a 'repulsive' energy — dark energy.
WHAT IS IT?
Unknown — possibly a property of space itself (cosmological constant, Einstein's 'biggest blunder').
Makes up ~68% of the total energy-mass content of the universe.
Composition of the Universe
CURRENT BEST ESTIMATES of the universe's content:
ORDINARY MATTER (stars, gas, planets, everything we can see): ~5%.
DARM MATTER (unknown, detected by gravity only): ~27%.
DARK ENERGY (unknown, causes accelerating expansion): ~68%.
Total matter and energy: 100%.
Only ~5% of the universe is made of stuff we understand — extraordinary.
IMPLICATIONS:
The ultimate fate of the universe depends on dark energy.
If dark energy grows: 'Big Rip' — universe tears apart.
If dark energy constant: universe expands forever, becoming increasingly cold and diffuse.
If dark energy decreases: universe might eventually collapse ('Big Crunch').
CURRENT RESEARCH:
Euclid space telescope, Dark Energy Survey — mapping distribution of dark matter and dark energy.
Large Hadron Collider — searching for dark matter particles.
Fermi telescope — searching for dark matter annihilation signals.
INTELLECTUAL SIGNIFICANCE:
The apparent 95% ignorance about the universe's composition is one of the great unsolved problems in physics.
New discoveries in this area could transform our understanding of fundamental physics.
⚠️ Common Mistake
Dark matter and dark energy are different: dark matter is a form of MATTER (has mass, interacts gravitationally) but doesn't interact with light. Dark energy is a form of ENERGY causing the accelerating expansion of the universe. Together they make up ~95% of the universe — neither has been directly detected or fully explained.