The first thing to point out is that we are entering a tremendously delicate terrain. If already talking about "ordinary matter" (that which we can measure and control) implies assuming the existence of quarks and leptons, of mechanisms of interaction between these particles by means of quantum force fields, much more confusing, blurry and even unintelligible is when we want to deal with the vacuum (from the quantum perspective), and the existence of the so-called "virtual particles".
We are in the realms of Quantum Field Theory, and outside mathematical language, all attempts to express in ordinary words what is happening, and, even more so, in an informative spirit, inevitably involve falling into forced arguments and approximations, always incomplete, and sometimes even erroneous.
There are physicists who are directly opposed, for example, to using the terminology of "virtual particle". They understand that it is best to be as faithful as possible to quantum field theory and not to consider them as particles at all, but as perturbations or excitations in a given field (such as the electromagnetic or gravitational field) forced by the presence of real particles or other fields.
We prefer to stick with the term "virtual particle" because from an informative perspective they have more advantages than disadvantages, and we are more coherent with the other subsections of this website.
In a classical field, the vacuum state contains nothing, is "inert" and has no waves or fluctuations. However, the vacuum state in a quantum field is continuously fluctuating with appearing and disappearing excitations, "virtual particles", fulfilling the Heisenberg uncertainty relation for the energy and the duration of the process: ΔE-Δt ≥ ℏ. That is, the more energy, ΔE, (or mass, Δm) of these appearing virtual particles, the less time, Δt, they "survive", disappearing immediately.

Recreation of quantum fluctuations (Taken from IFLScience)

Therefore, the quantum vacuum is not a place where there is nothing, only that the particles, fluctuations and energy there are so small and ephemeral that, for the time being, it is impossible to extract or transform them, but their "existence" is unquestionable. They are essential players in the interactions between common particles, as we indicated in another subsection, when we talked about Feynman diagrams, or in the establishment of the mass of particles through the Higgs field.
In addition, there are experiments based on the so-called Casimir effect, which show the presence of these quantum vacuum fluctuations, and an experiment is currently being carried out in Hamburg, using the world's largest X-ray laser (European XEFL), with the purpose of very precisely tracing quantum vacuum fluctuations.
On the other hand, in current cosmology it is accepted that most of the energy density in the universe is in an unknown form called "dark energy". It is thought to make up about 70% of the content of the universe (with dark matter, invisible stuff whose gravity pulls galaxies, making up 25%, and normal matter just 5%). Neither its origin nor its nature is yet known, but it is postulated that it is related to the quantum fluctuations of the vacuum, and would be responsible for the gravitational repulsion of all matter; and, therefore, for the accelerated expansion observed in the universe. If true, the ultimate fate of our world would be in the hands of these fleeting, "ghostly" virtual particles.
We can conclude this issue by saying that in classical physics the vacuum is an inert, inactive and "boring" place, but in quantum physics the vacuum is a very dynamic, busy and "fun" place.