We've been experiencing strange things in my house. Do ghosts really exist?

Basically some very strange things have been going on in my house. My mother has seen a girl on the stairs once & then a person in the kitchen twice. People who have come around have mentioned seeing movements & occasionally a shadow or sorts. My older sister who was recently visiting us from Washington saw a person walk from the kitchen to the pantry. She saw this in the reflection in the living room window while we were just hanging out & talking. It really creeped me out, but I wasn't quite sure if I believed in all that stuff or not until now. Just about 20 minutes ago I was in the downstairs bathroom combing my hair. My dog was in there with me & the door was closed. My dog started acting a bit strange & started smelling underneath the door. I heard footsteps walking across my living room floor toward the kitchen/bathroom area. The footsteps were a bit heavy sounding & I heard the floor creak & everything. A section of our flooring wasn't nailed down quite right so it creaks very loudly when you walk over it. As the footsteps got closer & louder my dog started growling loudly, putting his ears back & sticking his nose underneath the door. I said "hello?" & didn't get a response. I heard the footsteps on the linoleum kitchen floor & then just silence. I opened the door & nobody was downstairs except for me. It's 4 AM right now so my family was asleep except for my younger sister who was in the upstairs bathroom. It makes no sense whatsoever & now I'm stuck with this eerie/unsettled feeling.

What do you think of this? I don't know what to believe anymore. It's really creepy. My dog never acts like that unless he's protecting me from something. He would never do that if it were just my family. I live in an apartment building where a girl died in my living room while battling cancer & a man died in the unit next to me from a heart attack. I'm just looking for an explanation.

Also, we took a picture of my mother in the living room & in the picture there was this really weird silver/white mass on her chest. It looked almost like a flowing fog. It was taken with a digital camera & didn't show up in any of the other pictures taken only seconds before & after. It's just really weird.