So back to the problem: Where is the missing mass/energy?

If the universe is flat, then 70% of the mass/energy is not in the form of normal matter, dark matter, or photons. Then what is it?
The first clue: When Einstein was coming up with general relativity, there was an equation that needed an additional constant to make the universe static. He called this the cosmological constant. He also called it his biggest blunder.

If the universe is flat, then 70% of the mass/energy is not in the form of normal matter, dark matter, or photons. Then what is it?