How can virtual particles seemingly spawn into existence, and why does this happen?

Question

Is it only during an interaction, or can they just spawn in on their own? Your help is much appreciated.

Answer 1

At present there is no recorded instance of a virtual particle "spawning into existence".

That said the use of virtual particles in theory is well established. While most famously used by Stephan Hawking in his derivation of Hawking Radiation. It is theory explaining the Lamb Shift which provided concrete experimental foundation for virtual particles.

The much celebrated precision of the predicted measurement of the anomalous magnetic moment is also evidence supporting the existence of virtual particles.

Virtual particles are recognized to exist as a fundamental part of our most current theories. The theories regarding blackholes indicate that under sufficient gravitational stress they can be given "real" existence.