system
Given the following post, respond as if you're commenting on Reddit:
human
After the big bang why did some of the energy turn into matter?

I bet this is kind of a stupid question, but oh well.
gpt
Matter and energy are two sides of the same coin, when we're talking about "stuff" - photons, quarks, Higgs bosons, and all the other constituents of Nature. It's not as if the Universe was pure kinetic energy, for example (that wouldn't even mean anything - you need something to possess that energy).

Just after the Big Bang, the Universe was a soup of energetic particles running around and interacting. Some of these particles were massless, like photons, which you might think of as energy (though they're matter like anything else), and some of them were massive particles like protons and electrons. As the Universe expands, it cools, and as the Universe cooled after the Big Bang, the exact particle content changed, because when the temperature of the Universe dropped below the mass of a particle times the speed of light squared (divided by Boltzmann's constant), that particle would no longer be spontaneously created and so it would become much less abundant. So at the highest temperatures, when energies were higher than we can reproduce in the lab, we can only speculate about what kinds of particles existed, but after a little bit of time things became more familiar. By about a second after the Big Bang, the Universe consisted mostly of photons, neutrinos, and electrons (and their antiparticles, positrons), with some other things running around in much, much smaller abundances - protons, neutrons, dark matter, and so on.

Another question you might ask would be, if protons and neutrons and dark matter were so rare back then, why do we see so much of them today? The answer is that photons, being massless, *redshift* as the Universe expands, so that their energy density falls more quickly than the mass density of normal matter. So over time the density of matter becomes significantly greater than the density of radiation, as it is today.