Most of the helium in the universe formed

In 1929 the American astronomer Edwin Hubble discovered that the distances to far-away galaxies were proportional to their redshifts. Redshift occurs when a light source moves away from its observer: the light's apparent wavelength is stretched via the Doppler effect towards the red part of the spectrum. Hubble’s observation implied that distant galaxies were moving away from us, as the furthest galaxies had the fastest apparent velocities. If galaxies are moving away from us, reasoned Hubble, then at some time in the past, they must have been clustered close together.

Hubble’s discovery was the first observational support for Georges Lemaître’s Big Bang theory of the universe, proposed in 1927. Lemaître proposed that the universe expanded explosively from an extremely dense and hot state, and continues to expand today. Subsequent calculations have dated this Big Bang to approximately 13.7 billion years ago. In 1998 two teams of astronomers working independently at Berkeley, California observed that supernovae – exploding stars – were moving away from Earth at an accelerating rate. This earned them the Nobel prize in physics in 2011. Physicists had assumed that matter in the universe would slow its rate of expansion; gravity would eventually cause the universe to fall back on its centre. Though the Big Bang theory cannot describe what the conditions were at the very beginning of the universe, it can help physicists describe the earliest moments after the start of the expansion.

Origins

In the first moments after the Big Bang, the universe was extremely hot and dense. As the universe cooled, conditions became just right to give rise to the building blocks of matter – the quarks and electrons of which we are all made. A few millionths of a second later, quarks aggregated to produce protons and neutrons. Within minutes, these protons and neutrons combined into nuclei. As the universe continued to expand and cool, things began to happen more slowly. It took 380,000 years for electrons to be trapped in orbits around nuclei, forming the first atoms. These were mainly helium and hydrogen, which are still by far the most abundant elements in the universe. Present observations suggest that the first stars formed from clouds of gas around 150–200 million years after the Big Bang. Heavier atoms such as carbon, oxygen and iron, have since been continuously produced in the hearts of stars and catapulted throughout the universe in spectacular stellar explosions called supernovae.

But stars and galaxies do not tell the whole story. Astronomical and physical calculations suggest that the visible universe is only a tiny amount (4%) of what the universe is actually made of. A very large fraction of the universe, in fact 26%, is made of an unknown type of matter called "dark matter". Unlike stars and galaxies, dark matter does not emit any light or electromagnetic radiation of any kind, so that we can detect it only through its gravitational effects. 

An even more mysterious form of energy called “dark energy” accounts for about 70% of the mass-energy content of the universe. Even less is known about it than dark matter. This idea stems from the observation that all galaxies seems to be receding from each other at an accelerating pace, implying that some invisible extra energy is at work.

The first three minutes after the Big Bang is a time period worth of a whole book (indeed, such has been written). It is important because it was during this very short time period that all the hydrogen and most of the helium in the Universe today was formed. These are the two most abundant elements in the Universe, and are the basic materials from which the first galaxies and stars formed.

At the very instant of the Big Bang, the Universe formed as a rapidly expanding ball of energy at extremely high temperatures (trillions of degrees). As soon as it began to expand, it began to cool, and as it cooled some of the energy "froze" out in what physicists call a "symmetry breaking." This is similar to water freezing into ice, except in this case it was pure energy solidifying into matter. The conditions were right for about 75 percent of the "frozen" energy to become proton (hydrogen nuclei) and the rest to form combinations of protons and neutrons in the helium nuclei. This continued for only about 3 minutes, after which the Universe had expanded and cooled to the point that the process was no longer possible.

Thus, just some 3 minutes after the Big Bang began, the Universe was composed of hydrogen and helium nuclei with a great deal of leftover energy.

This seminal 3 minute period is populated with very short periods known as the Planck Era, the GUT Era, the Electroweak Era, the Particle Era, and the Era of Nucleosynthesis. However, the important thing to know is that it was during this very short time that the major building blocks for today's Universe were formed -- and have persisted to this day.

On final note may be in order. Most of the things around us in everyday life are composed of materials other than hydrogen and helium, although obviously there is much hydrogen in ordinary water, and protons (hydrogen nuclei) are part of all matter. But what about the heavier elements such as carbon, oxygen, silicon, iron and others? They were not formed in the Big Bang, so where did they come from?

After the first 3 minutes of the Big Bang, no further elements were created for millions of years, so the Universe was entirely hydrogen, helium and energy. However, eventually clouds of hydrogen and helium collected and contracted into stars under the force of gravity. In the cores of these stars nuclear fusion began and hydrogen fused into more helium, helium fused into carbon, and various other elements up to iron. For smaller stars, this is where it stopped. However, some of the most massive stars underwent enormous supernova explosions. In the supernovas conditions were right to form even heavier elements, all the way up to Uranium.

It is a sobering thought that every atom in your body can be traced back (in theory anyway) to the Big Bang, and the nitrogen and oxygen you breathe, the carbon your body is composed of all came from some ancient star. And the heavier elements, of which there are traces in your body, formed in the fiery furnaces of supernovas! No wonder the late Carl Sagan was fond of saying that we are all "star stuff."