This was a really nice surprise!
First and foremost, I really like the art style, the setting and the ambience. It's pretty oppresive and lonely, specially during the sections in the dark.
I really liked the gameplay and the puzzles. I had no problems with the filter system; it was actually really well made and it gave a sense of urgency and dread, so great job in that regard.
The music and the sound design were also really good, really fitting for the tone and ambience.
I think what I liked the least were some of the chase scenes, but they weren't really frustrating. I managed to get the 2 endings and my playtime was about 19 minutes. I liked the one where he drinks the Reptia, but the other one wasn't really my cup of tea. I would have loved more endings!
While I didn't really find the regular reptiles threatening, the giant one (was it subject 6?) was more intimidating and creepy, it kinda reminded me of Mr. Gray from The Dreamcatcher by Stephen King.

Overall, this was a reaaally good game, with a lot of attention to detail. I found no bugs whatsoever and I can tell you put a lot of passion and effort on this!

"THE DARK SUBSTANCE" es una experiencia de terror breve pero intensa. Me encantó la atmósfera opresiva y la historia intrigante que me mantuvo enganchada todo el rato. La jugabilidad es sencilla pero desafiante. A pesar de ser corto, ofrece una inmersión total, y estéticamente está muy guay. Muy recomendable para los fans del género. ¡MARAVILLOSO!

I'm mostly baffled that you would have this game out FOR FREE. It's incredible

Woah, this game was a real treasure to find! I love the artwork, and I love all the detail put into every little thing! If I didn't have a limited supply of resources, I'd love to just wander about and explore! The premise and details of the story are so eerie and cool to imagine- it feels like a real horror-sci-fi type film! (Like The Thing!)

Something about being stranded in a location with tons of hazards and unknown entities out to get you..and the time crunch to be able to mix and put things together for the vials etc. was challenging and felt like I really had to keep things prioritized in a cohesive way- I really enjoyed this entry!

From the horror aspect, to the creative design of, well, everything! Great work! :>

Wow the look and design of this one is so good!! It looks really polished and readable. The decaying oxygen and health mechanic was really clever and original! It worked, felt pretty fair but added an element of pressure. I don't love that the timer keeps going when you're reading a text box. I think sometimes that's okay, but if there's multiple instances of a lot of reading and it's all under a timer I just end up missing info cuz I'm skimming. But the writing was good, I felt like I knew what I was supposed to be doing all the time. I liked how dialogue changed on certain items as I progressed in the story, updating me on what to do next etc. (like look for the pick in the bunker type hints, super useful). Loved the look of it, very cohesive. Good job!! 

Really like the artstyle and the overall vibe!