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