who sings the song on the cbs promo for The Big Bang theory?

That song is called "Young Folks" by the group Peter, Bjorn, and John. It featured Victoria Bergsman of The Concretes as a guest vocalist.

Here are the lyrics:

if i told you things i did before

told you how i used to be

would you go along with someone like me

if you knew my story word for word

had all of my history

would you go along with someone like me

i did before and had my share

it didn't lead nowhere

i would go along with someone like you

it doesn't matter what you did

who you were hanging with

we could stick around and see this night through

and we don't care about the young folks

talkin' bout the young style

and we don't care about the old folks

talkin' 'bout the old style too

and we don't care about our own folks

talkin' 'bout our own stuff

all we care about is talking

talking only me and you

usually when things has gone this far

people tend to disappear

no one would surprise me unless you do

i can tell there's something goin' on

hours seem to disappear

everyone is leaving i'm still with you

it doesn't matter what we do

where we are going to

we can stick around and see this night through

and we don't care about the young folks

talkin' bout the young style

and we don't care about the old folks

talkin' 'bout the old style too

and we don't care about our own folks

talkin' 'bout our own stuff

all we care about is talking

talking only me and you

and we don't care about the young folks

talkin' bout the young style

and we don't care about the old folks

talkin' 'bout the old style too

and we don't care about our own folks

talkin' 'bout our own stuff

all we care about is talking

talking only me and you

(repeat)

talking only me and you

I don't know