What is the evidence for strong nuclear force?

The exchange particle for the strong nuclear force is the pi meson. It has been produced by high energy collisions in particle accelerators, and detected by means of cloud chambers and bubble chambers. There is also very persuasive indirect evidence for the existence of the strong nuclear force, insofar as no atomic nuclei other than hydrogen could exist without that force, given that positively charged protons repel each other, and would not remain in close proximity in an atomic nucleus unless there was some force stronger than the electromagnetic force, holding them together.