Funny scenes in The big bang theory

It was when Penny's ex-boyfriend came to a Halloween party in The big bang theory (season1), disc2.

Sheldon said,
"What do you suppose he's doing here,
...besides disrupting the local gravity field?"

Leonard responded by saying,
"If he were any bigger, he's have moons orbiting him."

The big bang theory, Lyrics of the theme song

I have watched an American TV show "The big bang theory" for a couple of weeks.
I like the starting music of the sitcom so I think I want to understand fully the lyrics.
(personal note: sitcom is the abbreviation (acronym?) of situation comedy)

I referred to this YouTube video.

Our whole universe was in a hot dense state,
then nearly fourteen billion years ago explosion started... wait!!
The earth began to cool, the autotrophs began to drool,
Neanderthals developed tools,
We built a wall (we built the pyramids).

Math, science, history, unraveling the mysteries, that all started with the big bang!

Since the dawn of man is really not that long,
as every galaxy was formed in less time than it takes to sing this song.
A fraction of a second and the elements were made.
The bipets stood up straight,
The dinosaurs all met their fate,
They tried to leap but they were late, they all died (they froze their asses off)
The oceans and pangea see ya wouldn't wanna be ya set in motion by the same big bang
It all started with the big bang!

It's expanding ever outward but one day it will pause then start to go the other way,
Collapsing ever inward, we won't be here, it won't be heard
Our best and the brightest figure that it'll make an even bigger bang.

Australopithecus would really have been sick of us
debating how were here
they're catching deer (we're catching viruses)

Religion or astronomy, Encarta, Deuteronomy
It all started with the big bang!

Music and mythology, Einstein and astrology
It all started with the big bang!