As a personal question, then the answers will be the same as the ones I have given to you before:
I don’t know that everything have always exist, and I don’t know if everything that exist and existed, have a beginning.
They are the same answers, whether it be personal or relating to the scientific or theoretical cosmology.
The scientific cosmology is that of the Big Bang cosmology, that only explained the evolution of the universe, from the still very and hot Planck Epoch and onwards.
1. But you need to understand that the timeline from Planck Epoch to the Baryogenesis, or to the end of the Inflationary Epoch, the model is still at the hypothetical and theoretical stage, and still not fully understood. And another thing is that nothing is observable, so these very earliest epochs have not being verified.
2. The timeline from the gluon-quark plasma to the 377,000 years after the Big Bang (where it also include the Big Bang Nucleosynthesis (BBN, only lasted from about 3 to about 20 minutes after the Big Bang) and matters become the dominant in the universe), the universe is still opaque, because matters are still ionised (with no electrons bound to the nuclei).
3. It is only from Recombination Epoch (starting at 377,000 years after the Big Bang), that the universe become transparent, because electrons stabilised matters (electrically neutral atoms). But this bonding between electrons and the nuclei of atoms, resulted in photon decoupling, and that’s what scientists can today observed, detect and measure, the freely travelled photons, the earliest observable light, which scientists called Cosmic Microwave Background Radiation.
Here, I described three main eras, before the formation of the stars and galaxies, which occurred around 1.5 million years after the Big Bang.
These eras described how particles (leptons, quarks first, before hadrons like protons and neutrons; hadrons comprised of 3 quarks) and later the bonding of protons and neutrons in the nuclei (in the the Big Bang Nucleosynthesis), followed by lot later with the Recombination Epoch, which I have already described as the 3rd era.
Eras 2 & 3 can be understood because of scientists knowledge of particle physics and quantum physics.
But all the various other models of cosmologies, for instance, about BEFORE THE BIG BANG, are all highly theoretical models, unsubstantiated (no evidences).
For examples, theoretical models, such as the Eternal Universe, the Oscillating Universe or Cyclical Universe model (known as the Big Bounce, where the universe underwent a series of birth, collapse, rebirth, eg Bang, Crunch, Bang), the various Multiverse models, etc. These are all still untestable and unverifiable, and the solutions are still at conceptual and mathematical stage, hence these models are still hypotheses.
My point in all this, it is impossible to know if the universe is eternal or not, but what I do currently accept, is the Big Bang theory that the describe the evolution of the universe, because they are currently the only cosmology that checked out.
