During the first 10 to the power (- 43) seconds [a decimal followed
by 42 zeros and then 1] all four forces (the strong and weak nuclear forces, the electromagnetic force
and gravity) were probably unified and behaved as a single force. This
period is known as the Planck era. Conditions during this era cannot be
described by known physics. At the end of this era, gravity and the other forces
of nuclear and electromagnetic became distinct.After about 10 to the power (-35) seconds the combined forces began to split into the strong nuclear force and the electroweak force. While this change was happening, the Universe Inflated, doubling in size every 10 to the power (- 34 seconds). By the end of this epoch, all distances had increased by at least 10 to the power (50) [1 with 50 zeros]
While the temperature was very high, radiation transformed into particle-antiparticle pairs and colliding particles and antiparticles transformed back to radiation. About 10 to the power (- 6) seconds [in other words one millionth of a second] alter the initial event, fundamental particles (quarks) formed protons and neutrons.
After about 100 seconds protons and neutrons fused to produce helium nuclei. The resulting ratio of helium to hydrogen was 25:75 (by mass). A hundred thousand years later, atoms formed, space became transparent, and the cosmic background radiation was released.
Eventually, galaxies began to form, perhaps a billion years after the initial event. Possibly, very large clumps of matter formed first and then fragmented into individual galaxies. Alternatively, small galaxies may have formed first, then coagulated into larger galaxies and clusters.
As the Universe expands, galaxies move further apart. Each galaxy "sees" every other galaxy receding with a speed proportional to its distance but no galaxy can claim to be the center. Every galaxy is receding from every other one, like dots on the surface of a balloon. (‘Science A History of Discovery in the Twentieth Century’, by Trevor Williams)