A mysterious quantity known as dark energy makes up nearly three-fourths of the universe. Yet scientists are unsure not only what it is but how it operates.
Another evidence for the Big Bang was provided by Edwin Hubble (1929) when he correlated the distance of objects in the universe with their velocities, using Hubble's law. Later the theorists of Big Bang used this law to find the age of Universe (14 billion years) which was consistent with the actual age of Universe.
The scientists believe that most of the universe is made up of dark energy, which is responsible for the expansion of the universe. Normal matter including galaxies, stars, comets, planets, etc could form only 5% of the universe, whereas 95% of the universe is only filled with dark matter.
We can have one more evidence for Big Bang which predicts the abundance of light elements like hydrogen, helium, etc. As per the nucleosynthesis of Big Bang almost a quarter of the universe should be filled with helium–4, which is in good agreement with what was observed.
Before the Big Bang there was no space and time which came into existence after it. So we need another theoretical model to explain what was there before Big Bang and what was the cause for the same.
Our Milky Way galaxy is one among 100 billion galaxies of the Universe. The truth of the cosmos is a mystery. The past discoveries about the universe cannot lead to any further exploration, as it keeps expanding. The cause of the universe would be some thing strange beyond our understanding and imagination.