Big Bang

The Big Bang is known for its beginning of this timeline. The Big Bang is totally unknown, so there is no proof on the Pre-Big Bang's history.

Heat
The heat of the Big Bang is hotter than the hottest star in the universe, which no-one survives the heat, surviving can cause death during this time. Even the strongest heat resistance elements, they could not survive and they melt entirely and boil.

It's temperature may be infinite.

Before the Big Bang
It may be nothing, or another universe that died during the great chill. It was completely unknown but we must find out later.