Our knowledge of the universe comes from looking at it in various ways — different wavelengths of light, neutrinos, cosmic rays, and hopefully some day gravitational radiation. But how do we know that we are seeing everything there is? How could we determine whether there were substances in the universe not directly visible in our telescopes?

Is the universe bursting at the seams? Or is nature somehow fooling us?

The astronomers whose data revealed this accelerating universe have been awarded the Nobel Prize for Physics.

And yet, since 1998, when the discovery was first announced, scientists have struggled to come to grips with a mysterious presence that now appears to control the future of the cosmos: dark energy.