Its direct observation is impossible.

It is too far away to be studied.

It is too small to be detected.

It is too weak to be measured.

Observing distant supernovae.

Measuring the cosmic microwave background.

Studying the large-scale structure of the universe.

Measuring the gravitational lensing of light.

Supernovae are too rare to be observed.

Supernovae are too faint to be detected.

Supernovae are too far away to be studied.

Supernovae are too variable to be used as standard candles.

The leftover radiation from the Big Bang.

The radiation emitted by dark energy.

The radiation emitted by the first stars.

The radiation emitted by the Milky Way galaxy.

By measuring its temperature.

By measuring its polarization.

By measuring its spectrum.

By measuring its intensity.

The distribution of galaxies and galaxy clusters in the universe.

The distribution of dark matter in the universe.

The distribution of dark energy in the universe.

The distribution of stars in the universe.

By measuring the redshift of galaxies.

By measuring the distance between galaxies.

By measuring the shape of galaxies.

By measuring the motion of galaxies.

The bending of light by massive objects.

The bending of light by dark energy.

The bending of light by the cosmic microwave background.

The bending of light by the large-scale structure of the universe.

By measuring the distortion of distant galaxies.

By measuring the time delay of light from distant galaxies.

By measuring the mass of distant galaxies.

By measuring the distance to distant galaxies.

Gravitational lensing is too weak to be measured.

Gravitational lensing is too rare to be observed.

Gravitational lensing is too variable to be used as a standard candle.

Gravitational lensing is too difficult to interpret.

Dark energy may help us to understand the origin of the universe.

Dark energy may help us to understand the fate of the universe.

Dark energy may help us to develop new theories of gravity.

Dark energy may help us to find new particles.

We do not know what caused the Big Bang.

We do not know what happened before the Big Bang.

We do not know what the universe was like at the moment of the Big Bang.

All of the above.

We do not know how the universe will end.

We do not know how long the universe will last.

We do not know what the universe will look like in the future.

All of the above.

We do not know how to incorporate dark energy into general relativity.

We do not know how to modify general relativity to explain dark energy.

We do not know how to develop a new theory of gravity that can explain dark energy.

All of the above.

We do not know what kind of particles dark energy is made of.

We do not know where to look for dark energy particles.

We do not have the technology to detect dark energy particles.

All of the above.