In space (space being used as the support for everything), there are tiny volumes where there is absolutely nothing. When this "nothing" is studied, it appears unstable. The more "nothing" you have, the more likely it is to spontaneously turn into "something" -- and the equivalent anti-something. In this way, the sum remains zero and no law is broken.
Almost always, when this is observed, the particles that are created are an electron and an anti-electron (also called a positron). Almost always, these two particles circle back to each other in an extremely small fraction of a second and cancel each other out (returning the nothing to nothing).
Such an event is a quantum fluctuation. Matter created out of nothing.
"Almost always" is not exactly the same as "always".
"Nothing" does exist and it appears to be unstable, wanting to break down into something (plus its anti-something).