Quantum fluctuations. That which cannot be measured by standard processes today is dark energy and David Bohm may have described it best in 1957 (Causality & Chance in Modern Physics pages 163-164):
“Thus, in the last century only mechanical, chemical, thermal, electrical, luminous, and gravitational energy were known. Now, we know of nuclear energy, which constitute a much larger reservoir. But the infinite substructure of matter very probably contains energies that are as far beyond nuclear energies as nuclear energies are beyond chemical energies. Indeed, there is already some evidence in favour of this idea. Thus, if one computes the “zero point” energy due to quantum-mechanical fluctuations on even one cubic centimetre of space, one comes out with something of the order of 1038 ergs, which is equal to that which would be liberated by fission of about 1010 tons of uranium.”
A few references with this website:
Worldviews limit perspective 