We have pondered this question, here at Deskarati, for some time and have read many accounts and theories over the years. The answer we have decided is that ‘nobody knows’. So we decided to re-read all available ideas again and after much searching we find that the following short explanation by ‘Kurious’ on the Physicsforum comes closest to our own best opinion on the subject:
Dark matter is considered to be made from exotic particles like WIMPS (WEAKLY INTERACTING MASSIVE PARTICLES) or MACHOS or it could be made from other undiscovered exotic particles. Dark matter might account for the extra gravity that allows stars to have higher velocities than normal in spiral galaxies and that allows galaxies to orbit one another at speeds far higher than Newton’s laws allow. (Modified Newtonian Dynamics, MOND, a theory by Mordechai Milgrom, changes Newton’s force law and accounts for experimental data very closely indeed but no-one knows why MOND works!).
Dark energy is considered to exist everywhere in the universe and to be more energy like than mass like – hence the name dark energy . Many theorists believe dark energy is a scalar field called quintessence (there is also a version called spintessence). Dark energy has a constant energy density throughout the universe and the total amount of dark energy increases as space expands (some people think the extra dark energy is made from gravitational potential energy). Dark energy appears as the cosmological constant in Friedmanns’s equations and has negative pressure – it is gravitationally repulsive and it is causing the expansion of the universe to accelerate . Observations of the brightness of the same supernovae at different times have confirmed that the expansion is accelerating-the supernovae are dimmer more than expected after a fixed time has elapsed, so they are further away than can be explained by a uniform velocity.
It is not known for sure that dark matter exists and the evidence for dark energy is convincing. Dark energy may be dark matter in some theories. Most of the mass of the universe is considered to be in the form of dark energy – at least 70 per cent.