Because we can't measure it directly, we don't even know what it is made of, then formulating experiments to detect it and study its very nature is really challenging. Also, current observations disagree with the value of the Hubble rate at the present time, therefore, we are uncertain if dark energy is changing in time, and if so, how it is affecting the dynamics of the expansion. We have clues, but there is still a long path to go before "unveiling" dark energy's nature and traits.