There is no observational information at all. There is no scientific data or observation that has ever been registered, or can ever be obtained, that could demonstrate that 'nothing' existed before the Big Bang. All we can do is draw inferences from what little data we have, and make the best guesses we can from our best theories of how the physical world operates.
What we would like to do is to observe what happens when you concentrate energy in the collisions produced in a particle accelerator, to those that prevailed soon after the Big Bang. Right now, we can manufacture collisions with energies of about 2000 billion electron volts. The rest mass energy of a proton is about 1 billion electron volts by comparison. To test some of the 'modest' extensions of our best theories of high-energy physics we want to observe collisions at energies of 10,000 to 20,000 billion electron volts. At these energies, some of the extensions of the 'Standard Model' predict that electrons and quarks may show themselves to be built from composite particles, other extensions based on 'supersymmetry' predict that completely new types of sister particles to quarks, electrons and photons will begin to appear in the reactions. We should also, by these energies, begin to see the appearance of the so-called 'Higgs boson' which has produced the symmetry breaking between the electromagnetic and weak forces at lower energies. If the Higgs boson never appears, this will be a dramatic moment in physics because the concept of symmetry and symmetry-breaking moderated by these 'bosons' is THE MAJOR PARADIGM of 20th century theoretical physics.
Anyway, to begin to answer questions about the Big Bang itself, and to test those theories that claim to discuss this event quantitatively, we need to verify all of the rest of the physics between energies of 20,000 billion electron volts, and 1000 trillion billion electron volts, or 10^15 GeV. This takes us to the so-called Inflationary phase transition with its own essential compliment of superheavy particles. But even this energy is not high enough to really test 'dimensional compactification', 'super string theory' and 'quantum space-time dynamics' which occur at 10^19 GeV or 10,000,000 trillion billion electron volts. Without the data to confirm the direction that theories take between 10,000 trillion electron volts and 10^19 GeV, we can only make our best theoretical guesses. However, some of the 'biggest minds' in quantum mechanics and general relativity, assert over and over again that even the simplest possible reading of general relativity shows that space and time only came into existence 'after' the Big Bang event itself. These 'parameters' of the physical world had no meaningful existence 'prior' to the Big Bang. At the Big Bang itself, spacetime was a quantum mechanical 'system' in which cause and effect and perhaps even space and dimension were not meaningful concepts.
It is not that 'nothing' existed 'before' the Big Bang, only that the terms we use to organize our thinking about this 'state'; terms such as time, duration, place and size, were not any more meaningful than 'spin', 'wave', 'particle' 'color' are to us today. Just as the concept of 'quantum mechanical spin' has no meaning outside the atom, the presumption is that the concepts we need to discuss what happened 'before' the Big Bang may be just as illusive. Until our theories can lead us step-by-step to the right set of organizing elements, we can probably not think of the Big Bang as anything more that an slate upon which we cannot write. Until physicist invented the idea of 'spin', for example, there was no way to classify particles properly. Until theory and experiment point us in the right direction, we will not have the right words to describe the Big Bang. Remember, we are not dealing with science fiction here. Words in science have very specific mathematical usages. You cannot use the term 'spin' in any other context than the one in which the theory of quantum mechanics directs you. Big Bang physics at energies of 10^19 GeV will most certainly have to have its own very special dictionary to take us from mathematics into the data of the physical world.