Cosmos Era endings guide: how to unlock all 16 endings

The short answer: for most of the 16 endings, the pattern is "unlock a certain node, then witness the end of the universe". Three need other endings first (Baby universe, Recursion, They can see you). Three need a specific choice in a specific era, or a count to be met (Words in the sky, The trilobites' civilization, The microbes won). "Proton decay" needs 24 hours of real-world waiting. The last one, "You", needs you to witness the other 15.

Terms

Witness the end of the universe: push the main line to the last event of the last era.
Unlock: unlock a node in the evolution tree.

The 16 endings one by one

1. Time loop (Spacetime)

Hint: At the end of the spacetime constellation, there is a letter addressed to the past.
How to unlock: unlock "Message to the start", then witness the end of the universe.

2. What dark matter is (Physics)

Hint: Dark matter gives off no light and says nothing. Perhaps it is only waiting for someone who will listen.
How to unlock: collect all seven dark matter research cards, unlock Dark matter halos, Dwarf galaxy, Galaxy cluster and Gravitational lens, then unlock "Talking to dark matter" and witness the end of the universe.

3. Words in the sky (Sky)

Hint: When the first star lights up, look up at the sky event of that moment.
How to unlock: witness the main-line event "First stars" under 5 different cosmic sky events.

4. Rewind (Physics)

Hint: What if dark energy is not constant?
How to unlock: unlock "Big Crunch", then witness the end of the universe.

5. All the planets (Mind)

Hint: Your album keeps getting thicker. If those planets could think together...
How to unlock: have at least 30 planets in the album, unlock "Cosmic mind", then witness the end of the universe.

6. The other end of the wormhole (Spacetime)

Hint: A wormhole can only lead to places you've been. So why does the sky at the other end look so familiar?
How to unlock: unlock "Through the wormhole", then witness the end of the universe.

7. Vacuum decay (Physics)

Hint: Our vacuum may only be metastable.
How to unlock: unlock "Triggering the vacuum", then witness the end of the universe.

8. Silence (Information)

Hint: In the dark forest, the safest sound is no sound.
How to unlock: unlock "Permanent silence", then witness the end of the universe.

9. Baby universe (Energy)

Hint: Watch this universe die first, then decide whether to build a new one.
How to unlock: first witness the ending "Proton decay". Then unlock "Cosmic factory" and witness the end of the universe.

10. Recursion (Information)

Hint: Civilizations inside a simulation make games too.
How to unlock: first witness the ending "All the planets". Then unlock "Video games" and "Recursion", and witness the end of the universe.

11. They can see you (Information)

Hint: Every time you look, the universe becomes a little more certain. Someone is bound to notice.
How to unlock: first witness the ending "All the planets". Then unlock "Seeing the Observer" and witness the end of the universe.

12. They moved into your device (Mind)

Hint: The nearest computer may not be among the stars.
How to unlock: unlock "Moving into the device", then witness the end of the universe.

13. Proton decay (Physics)

Hint: Some endings you can only get by actually waiting.
How to unlock: unlock "The last proton", witness the end of the universe, then wait 24 hours in real life.

14. The trilobites' civilization (Life)

Hint: One creature nearly vanished in each of three mass extinctions. What if someone protected it every time...
How to unlock: in a single run, protect Trilobites (or a sub-group of them) through all three mass extinctions: End of the Ordovician, Late Devonian and End of the Permian. Keep Trilobites alive until Human origins, which means protecting them in the later mass extinctions too. If "Memory echo" skips those eras, you can lower it in "Upgrades".

15. The microbes won (Life)

Hint: What if life hid in the deep when oxygen arrived, and never grew animals...
How to unlock: choose "Hide in the deep" when you face the Great Oxidation Event, don't unlock Collagen or Animals, and reach Human origins. If "Memory echo" skips those eras, you can lower it in "Upgrades".

16. You (Ultimate)

Hint: Witness all the other endings.
How to unlock: witness the other 15 endings.

A suggested order

Start with the one-step endings: Time loop, Rewind, The other end of the wormhole, Vacuum decay, Silence, and They moved into your device. Each is one node unlocked, then witnessing the end.
Then the ones that need collecting: What dark matter is (seven dark matter research cards) and All the planets (at least 30 planets in the album).
Then the ones that need another ending first: Recursion and They can see you both need "All the planets", and Baby universe needs "Proton decay".
Plan these separately: Words in the sky (witness the first stars under 5 different sky events), The trilobites' civilization, The microbes won (specific choices in the right eras), and Proton decay, which needs 24 real hours.
Last: You.

A few reminders

"The trilobites' civilization" and "The microbes won" need specific choices in specific eras. If "Memory echo" skips those eras, you can lower it in "Upgrades".
"Proton decay" really does make you wait. Unlock "The last proton", witness the end, then wait 24 hours in real life. You can leave the game and do something else.
The hints are not decoration. They point at the unlock conditions, so when you're stuck, read them again first.

(Sources: the n, cat, hint and how fields of each ending in the game data TREE.D.ENDINGS.)