Here is the full list of all 26 The Dark Side of the Moon achievements.
You attacked Julian!
You got ending number 1!
You said that you believe in God.
You got ending number 2!
You trusted Blackwell over Alyx.
You made Gedeon reveal himself.
You shouted at the homeless man!
You offended Blackwell.
You got Dean killed for real.
You found an Easter Egg!
You made it to the end...or did you?
You got ending number 3!
You stopped Gedeon!
You called Alyx at the right time!
You made it to the moon!
You discovered Sarah's secret dungeon!
You called Alyx while with her.
You had a catchup with Julian.
You trusted Julian.
You found the homeless man dead.
You trusted Alyx with no real hesitation.
You said that you don't believe in God.
You gained access to the kids' laptop.
You gave the homeless man money.
You said goodnight to the kids first.
You took out the rubbish first.