Question

The theory says that the big bang produced the chemical elements hydrogen and helium. What evidence supports this?

Answer 1

The big bang is a bit complicated.

Now, Kurzgesagt does an amazing video on this (and yes, I know mods dont like links very much) : https://www.youtube.com/watch?v=wNDGgL73ihY

I will explain as best I can.  So, lets assume that the big bang theory actually happened for right now before any debates arise.  First of all, we have no clue what even started it.  There are so many theories as to why it started and was their a universe before the big bang.

Now, this is the interesting part.  Every universe has its own laws.  We have four laws or forces in ours that we know of (and then we have quantum physics which completely messes with physics but we wont get into that).

The Force of Gravity

The Strong Force

The Weak Force

The Electromagnetic Force

We have no clue if there are any other universes and if they are, do they have different properties.  If so, chemical bonding could be completely different.

So much could be different.

Now, we know there are also some laws like conservation of energy and law of balance.  Balance is what made the universe.  Atoms had to find their "mate" lets just say and create chemicals.  These chemicals would then go on to bind more and more and create other things over time as the universe was also expanding.

The universe is still expanding (and even accelerating its expansion).

Things must be chemically stable and thats how bonds are formed.

Now, this also kinda opens up "why" and science does not always answer "why".  Thats more for religion or philosophy.

Its very interesting and I suggest you look into the video I linked.

Hope this helps

Answer 2

One of the biggest evidence that big bang produced hydrogen and helium is the Cosmic Background radiation proof. CBR is basically the matter and antimatter energy released  during the Big Bang. The evidence of CBR are apparent as a radio signal which have a temperature of 2.7 K. Also, through extensive spectroscopic surveys  from distant stars and galaxies , it is found that hydrogen and helium  make up  all the matter in the universe.  Hydrogen accounts for 74%  while helium contributes 25% of the total mass.