Nor could the virus be detected directly using the best available tests.

Like dark energy, dark matter has never been detected directly.

Important as it is, the Higgs field cannot be detected directly.

The serious study of gravitational waves - which have not been directly detected yet - is even further in the future.

But the particles on the list are typically not the ones directly detected by instruments.

The researchers still cannot directly detect an electron's jumping into an empty space.

They are known to be everywhere throughout the universe, and yet neutrinos have never been detected directly.

The current due to each electron can be directly detected experimentally.

This is the same kind of energy we see as light, but at frequencies we cannot detect directly.

The quarks themselves exist for only a fraction of a second, and cannot be detected directly.