An extensive and varied ammount of music was used in the original broadcast including works by Edgar Froese (Tangerine Dream), Brian Eno, Pink Floyd and Jean Michel-Jarre. However, it is the 3rd movement of Heaven and Hell by Vangelis which is most recognised. It is this, which was used as the main theme.
Taken from the album of the same name, the 3rd movement is often called Theme from Cosmos on compilation albums (for example The Gift) as it was on the 7" single release.
Confusingly Vangelis also wrote a new track for "the special edition" which is also called Theme from Cosmos, but this has never been released.
Here then is a full listing of all the Vangelis music used in the original series.
This is not the list for the Turner re-broadcast or the video cassette releases. Rather than get the proper rights for the re-issues, they comissioned a new score.
Show #1: HEAVEN & HELL CREATION DU MONDE fr L'APOCALYPSE DES ANIMAUX Show #2: ALPHA fr ALBEDO 0.39 HIMALAYA SUMMIT HEAVEN & HELL IGNACIO Show #3: HEAVEN & HELL Show #4: CHINA - HIMALAYA SUMMIT HEAVEN & HELL IGNACIO Show #5: HEAVEN & HELL IGNACIO Show #6: HEAVEN & HELL Show #7: HEAVEN & HELL CREATION DU MONDE fr. L'APOCALYPSE DES ANIMAUX Show #8: HIMALAYA SUMMIT fr. CHINA PULSAR - side 1 band 1 fr. ALBEDO 0.39 PULSAR - side 1 band 4 fr. ALBEDO 0.39 ALPHA fr. ALBEDO OP. 39 HEAVEN & HELL Show #9: ALPHA fr. ALBEDO 0.39 HEAVEN & HELL SPIRAL - SPIRAL Show #10: HIMALAYA SUMMIT fr. CHINA THE LITTLE FETE fr. CHINA HEAVEN & HELL CREATION DU MONDE fr. L'APOCALYPSE DES ANIMAUX Show #11: BEAUBOURG PART II HIMALAYA SUMMIT fr. CHINA HEAVEN & HELL Show #12: HEAVEN & HELL BEAUBOURG, PART II ALPHA fr. ALBEDO 0.39 IGNACIO CREATION DU MONDE fr. L'APOCALYPSE DES ANIMAUX Show #13: ALBEDO 0.39 BEAUBOURG, PART II HEAVEN & HELL IGNACIO
My thanks to Dennis Lodewijks firstname.lastname@example.org for additional information and finding the original cue sheet listing.