When people talk about evidence for the Big Bang, they're most likely to point to the Cosmic Microwave Background. But there's another major piece of evidence that the Big Bang happened, and it involves the element that makes your voice squeaky. This is the story of how helium can show that the universe had a definite beginning.
The production of helium inside the stars has been understood for quite some time. Protons are squashed into each other to form heavier and heavier elements. In the case of the helium in the sun, two protons form a deuterium atom. The atom is then smashed with another proton to make a helium-3 atom. When a second helium-3 atom collides with the first helium-3 atom, two protons wander off on their own, and the remnants of the two atoms combine to make a helium-4 atom, with two protons and two neutrons. This is the most common type of helium atom, and in the sun - in any of the stars - its production liberates a lot of energy. Stars shine because this type of thing is happening in them.
After figuring out how helium production worked astronomers were a bit puzzled. They'd also found that twenty-four percent of the visible atoms in the universe were helium. After looking at their figures and looking at the sky, they turned to each other and wondered if it wasn't a bit dark in the universe for that much helium having been produced. And, considering the age of the stars, they shouldn't have that much helium in them at all. Nothing matched up.
Until someone considered the idea that perhaps helium production wasn't just happening piecemeal in the stars. Perhaps it had happened at some point when the entire universe, everything that now existed, was under a lot of pressure, close together, and generating a lot of heat. During the first part of the Big Bang, the universe was too energetic for even protons and neutrons to form. But after they did, and while they were all still piping hot and relatively densely packed, they must have smashed into each other and produced helium. The phenomenon was named Big Bang Nucleosynthesis, and it's the only way, as far as anyone knows, that so much helium could have been formed throughout so much of the universe.
Interestingly, the helium in the balloons that you use to decorate your birthday party isn't proof of the Big Bang. That was mostly made by decaying uranium. When uranium decays, it spits out an alpha particle - two protons and two neutrons. The particle picks up a couple of electrons and becomes helium. Most helium harvesting facilities are right next too uranium stores. Sadly, you have to look into space to see the helium that's left over from the Big Bang.
Top Image: D Sharon Pruitt