Question

Measurement of the sizes of hot spots in the temperature distribution of the cosmic microwave background suggests that the universe is flat. Yet, the combined density of all the matter and energy we see, plus the dark matter we infer, is only about a quarter of the critical density. What conclusion have we drawn from this discrepancy?

Answer 1

There is an entity to which we have given the name dark energy which contributes to the total density but which we cannot detect by the usual methods.