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