Dark Energy & Dark Matter (OCR A Level Physics)

Revision Note

Test Yourself
Katie M


Katie M



Dark Energy & Dark Matter

Dark Energy

  • Scientists know that the universe is accelerating as it expands
  • Dark Energy is a hypothetical form of energy which is used to try and explain the accelerating expansion
  • Dark energy is defined as:

A type of energy that permeates the whole universe and opposes the attractive gravitation force between galaxies via the exertion of a negative pressure

  • Dark energy cannot be detected directly
    • It should make up 68% of the total energy in the universe
    • So far experiments have not been able to find the form of the energy

Dark Matter

  • Astronomers expect to observe the velocity of an object within a galaxy decrease as it moves away from the galaxy’s centre
    • This is thought to be the case because of weakening gravitational field strength further from the centre
  • This is observed in smaller mass systems, such as the in the solar system, where planets orbiting furthest from the Sun have the slowest orbital velocity
    • This is not the case in larger mass systems, such as entire galaxies


  • In fact, mass is not concentrated in the centre of galaxies - it is spread out
  • However, all the observable mass of a galaxy is observed to concentrate in the centre of galaxies
    • Therefore, there must be another type of matter that can't be observed
    • This is known as Dark Matter
  • Dark matter is defined as:

Matter which cannot be seen and that does not emit or absorb electromagnetic radiation

  • Dark matter cannot be detected directly through telescopes
    • It is estimated to make up 27% of the mass in the universe
    • It is detected based on its gravitational effects relating to either the rotation of galaxies or by the gravitational lensing of starlight

You've read 0 of your 0 free revision notes

Get unlimited access

to absolutely everything:

  • Downloadable PDFs
  • Unlimited Revision Notes
  • Topic Questions
  • Past Papers
  • Model Answers
  • Videos (Maths and Science)

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Katie M

Author: Katie M

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

Join over 500 thousand students
getting better grades