Half life ( if you mean it in a scientific way)
Means the time taken for half the atoms in a
Radioactive substance to decay.
And in reference to why there was only hydrogen in
The early universe is that helium and hydrogen existed
Because all other heavier elements were pulled together
Forming stars. Only when the stars exploded did heavier more
Metallic elements were formed through fusion. There's a lot more
To the why only hydrogen and helium existed but its the gist of it.