Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

We live in a dark world, and the light is results of big bang. The world is really made mostly of dark matter and dark energy? Why the world is so deep and dark?

Dark world

share|cite|improve this question

If you're asking why baryonic matter only makes up 4.6% of the matter in the universe then there isn't an answer for that.

Measurements of the Cosmic Microwave Background tell us that space is flat (to within 1% or so), and general relativity tells us that if space is flat it must contain a specific matter/energy density, called the critical density. In fact the current estimate of the critical density is about 5 hydrogen atoms per cubic metre, or about $10^{-26}$ kg per cubic metre so the critical density isn't very dense.

So on the one hand experiment (plus GR) tells us there must be $10^{-26}$ kg per cubic metre out there, but when we count stars and galaxies and estimate their mass we can't see anything like this much matter. in fact the visible matter we can see makes up only about 2% of the critical density. However recent discoveries of clouds of atoms surrounding galaxies suggests we may now have found the remaining 2.6%.

So on the one hand experiment tells us there must be $10^{-26}$ kg per cubic metre in the universe, but on the other hand experiment tells us there is only about $5 \times 10^{-28}$ kg per cubic metre. The difference is made up by dark matter and dark energy.

But we don't know why dark matter and dark energy make up the remaining 95.4%. We just observe that they do. Maybe one day some theory of everything will tell us why this is the case, but at the moment no-one has any idea.

Incidentally, the reason your two pie charts for now and just after the Big Bang are different is that as the universe expands matter gets more dilute, but the dark energy density stays the same. That means as the universe ages (and expands) dark energy is a bigger and bigger fraction of the matter/energy in the universe. Conversely when the universe was very young and the matter density was very high, it completely swamped the dark energy.

share|cite|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.