Years ago my little sister was choking on her food. She was coughing heavily and my mother had to pat her on the back. Because I knew it was serious I walked into the room. As I came in the room filled with darkness. It was like a dark fog falling from the ceiling with the speed of a feather that floats to the ground. The occurence lasted about 2 seconds. I guess it was over when my sister was fine again. No one but me saw it. Sometimes I think of this and now I googled if anyone made a similar experience. I found the report of a nurse saying she saw a dark, thick rolling fog when she entered the room of a patient and she too thought it was death. She said the patient died later that day. I can link the source if wanted. My sister is still alive. Maybe death was there because it expected to get her. If it was death. I never saw this before and never again after. I don't know if it was death but I know that what I saw was real. I want a scientific explanation. I was not in a panicking or scared state, just worried. My mind was clear, not under the influence of an adrenaline rush, medicine, drugs, alcohol, sickness or anything else.