Has the spontaneous forming of matter been observed?

Question

If I understood the accepted answer here correctly,then matter can spontaneously form. But has this ever been observed?

Answer 1

The answer you cite is somewhat simplified to suit the level of the question. The vacuum does not consist of pairs of particles and anti-particles popping into existence and then disappearing again. When calculating the properties of the vacuum it's true that we use Feynman diagrams showing the creation of particle/antiparticle pairs, but these are virtual (anti)particles that are a computational device. They do not mean particles are literally appearing from nothing.

Virtual particles cannot be observed but their effects can. For the vacuum the obvious evidence is the Casimir effect. Since this has been experimentally measured you could regard it as an observation of virtual particles appearing and disappearing, however I must emphasise that the virtual particles are not really particles in the common sense of the word.