This song, i feel, is about how Greg feels that total comprehension is beyond man's, or at least his, understanding. "The seeds of inspiration never germinated in my mind," i feel is truly poetic and a great way of putting his inability to comprehend "what 'it' is". "The beacon of awakening," which i believe is essentially enlightenment, "is somewhere i can't find."
I believe 'it' is an ultimate Truth of the universe, and explanation to it all, something that rivals God.
Just because Greg doesn't have an answer, doesn't mean he is wrong. He can make sense of "all of this," but he doesn't know where it came from and what it means.
true answer to the universe, the reason for our life, and the galaxies beyond this

think it might be the same (ubiquitous) thing throughout the song. Could it be life, our existence? Striving for some meaning in this life? Trying to find out if/what's the reason for all of this?

The first problem one encounters by trying to decipher what Greg was trying to say in this song is what 'it' is. It's used all over, but Greg never says what 'it' really means. 'It' continuously changes meanings, so it's hard to define. The songs talk about the meaning of life, in general, but that this 'it' (the meaning) is unknown. He says, 'there's a purpose, there's a goal . . . there's a reason for all of this . . . there's a beginning and there's an end, there's a climax.' All is trying to explain why humans are here, why there's life, it's kind of religious (but we know Greg's position towards religion, so I wouldn't consider that strongly). By there's a 'climax,' Greg means that there's a stage in your life when you're either in the best or worst position, when one move will determine your whole life. There's a beginning to life, then the climax comes, and finally there's an end--death. There might be a reason for all the suffering and pain and rewards in life, but Greg simply can't describe what this 'it' is. No one can, and that's why this song is tough to define.