At present, mainstream physics posits that the underlying level of all natural phenomena is quantum mechanical.
There exists the standard model of particle physics, a mathematical model, where it is posited that the elementary particles and their interactions build up matter as we know it, classical theories emerging smoothly from the underlying quantum mechanical framework.
Quantum mechanics is a probabilistic theory, the elegant mathematical formulations can only predict a probability for the location and time of an event, given by the solution of the quantum mechanical equations. For example, there are no orbits of electrons around atoms, but orbitals, probability loci.
For mainstream physics, a measurement is a specific instance manifested out of all the possible values that the probability distribution ( the complex conjugate square of the wavefunction) offers, misleadingly called "collapse of the wavefunction".
There exists the so called "many worlds interpretation"
The many-worlds interpretation is an interpretation of quantum mechanics that asserts the objective reality of the universal wavefunction and denies the actuality of wavefunction collapse. Many-worlds implies that all possible alternate histories and futures are real, each representing an actual "world" (or "universe"). In layman's terms, the hypothesis states there is a very large—perhaps infinite2—number of universes, and everything that could possibly have happened in our past, but did not, has occurred in the past of some other universe or universes.
It is not the mainstream view. It is interpreting mathematical functions and integrals as a version of reality, but there is no way that this can be checked by experiment.
It is the old conundrum: does mathematics define nature or nature is modeled by mathematics? Started at the time of Pythagoras and Plato, and this is its latest manifestation.