https://www.vice.com/en_us/article/3az8g3/ok-wtf-are-virtual-particles-and-do-they-actually-exist

Last June, Boston University professor Gregg Jaeger travelled to Växjö, Sweden for a conference. It was the twentieth time that philosophers had gathered there to discuss questions that strike at the foundations of physics. Jaeger had been invited to give the opening talk, to speak about mysterious and sometimes controversial entities called “virtual particles."

Whereas matter had long since been recognized to be made up of particles, the existence of virtual particles had been debated by philosophers of physics for at least thirty years. Mostly, they leaned towards their dismissal, but Jaeger is a believer.