The universe is full of matter, and matter has (positive) energy. This energy, however, is - possibly precisely - balanced by the negative gravitational energy of everything pulling on everything else. In other words, the total energy of the universe may be zero!
The idea of a zero-energy universe suggests that all one needs is quantum fluctuation – a ripple in nothing. The Universe could have created positive energy of matter balanced by negative gravitational energy, which precisely cancel each other out, but allow the Universe – with all it's stars and galaxies and life, to exist without violating the Law of Conservation of Energy.
The meaning of nothing is itself a very large question in quantum physics. Empty space is seething with particles and antiparticles which are born out of nothing, live brief lives and die again by annihilation with their antiparticles without violating the Law of Conservation of Energy because they are born from no energy and return to a state of no energy.
These particles may have lifetimes described by Heisenberg's Uncertainty Principle, one form of which is
This form of Heisenberg's Uncertainty Principle says that the smaller the uncertainty in the energy, the longer the lifetime of that energy state. If
as may be for the Universe, then
may be infinity or at least very long.
Perhaps many quantum fluctuations occurred before the birth of our universe. Most of them quickly disappeared. But one lived sufficiently long and had the right conditions of matter creation and gravitational energy. Thereafter, the original tiny volume expandedr, and our macroscopic universe was born. The original particle-antiparticle pair (or pairs) may have subsequently annihilated each other – but even if they didn’t, the violation of energy conservation would be minuscule, not large enough to be measurable.
If this admittedly speculative hypothesis is correct, then the answer to the ultimate question is that the universe is the ultimate free lunch! It came from nothing, and its total energy is zero, but it nevertheless has incredible structure and complexity. In fact this could have happened many times, maybe even within our own Universe.