Unfortunately in need of opinions regarding cause of death

I posted this on another website but haven't gotten any responses, so I'm going to copy & paste it here.

About 7 months ago I picked up a monster of a cat from Petco. His paperwork was very limited, but he was an owner's surrender (reasons unknown) and estimated to be around 3. Breed seems to be Chartreux - really huge, grey (shiny, soft, short coat), lazy cat. Clean vet bill.
He had his moments of high activity & play, but seemed to really enjoy the luxury of being a cat and sleeping in the sun most of the time. No symptoms or cause for alarm at any point, he was always verbally chirpy and happy.

Last night he came crashing into my room, as he often does during his spurts of high-energy play, but it was different. He stopped for a moment, let out a very deep, bellowing meow that sounded more like a moan, then went under my bed. He did it a few more times and then was silent. I looked under & saw he was against the wall, so I got up and pulled the bed out to see him. He was face-down on a blue sheet with a small pool of clear liquid soaked around it (probably saliva and nose-run). I was immediately alarmed and called his name, then pet / poked him. He was unresponsive so I gently scruffed his neck & pulled his face up, and his eyes were nothing but black - his pupils were dilated so extremely that there was only a sliver of color. My heart sank when I saw his eyes, I knew this was now something very serious. I let go of the scruff and his head just plopped back down. I nudged him a bit more and then fully scruffed him and pulled him out. His tongue was out of his mouth and he was already dead. No breathing or response to anything - no pulse when I held my hand on his body. It all happened in just a matter of minutes.

Does anyone have any idea what it could have been? My thought (after some research) is either heart attack or brain aneurysm - I don't know what else could have caused such an instant death without any preceding symptoms. Are the extremely-dilated pupils an important sign as to what it might have been? Or is that just a common stage of death in cats? I just want some sort of closure, I feel terrible. Thank you in advance for any help

First, I am so very sorry to hear about your cat! I've been in the same situation of having a cat pass unexpectedly at home and just wanting to know WHY?!

Yes, dilated pupils are common postmortem.

I too was thinking something heart related. Hypertrophic Cardiomyopathy can be present without any obvious symptoms (such as a heart murmur).

I am not sure if you could get an accurate answer at this point; but, your vet can preform a necropsy (animal autopsy) to see if there are any obvious causes of death.

Another thing it may have been (though rare for a young cat) is a hemangioscarcoma.

Our cat passed of a very rare degenerative brain disease at the age of 3.5. A full necropsy was the only way we were able to identify this. But, we were concerned that he may have had something genetic that his siblings could have as well.

I'm not sure if this is any help, but my daughter's one-year-old passed in mid-stride many years ago, and the vet said it was a "stroke" i.e. ruptured aneurysm.  It's heartbreaking when that happens, but at least since you were there, you know he went quickly and didn't suffer.  I'm glad he experienced having been adopted and having a new home before he left.

In December, this happened with my cat Estelle. She wasn't at all active as a cat, but had a really good day and seemed fine. She was sitting under the Christmas tree and then went under an end table, meowed loudly and my fella helped her out from under the table. She sort of stared off and then her legs just spread out from under her. She was unconscious and barely breathing. We took her to the emergency vet (of course it was late at night). And after tests galore, they said she was declining super fast and that it seemed neurological. They could only recommend that we take her to a more specialized vet hospital in Tacoma. We chose instead to end her suffering. By that point...I think she would have gone on her own in minutes. It really seemed similar to what you describe.