What is existence?

[VGPolyglot]

So, a huge question that always pops up is how do we exist, or why do we exist? However, what is existence itself? A famous term is "Je pense, donc je suis", or in English "I think, therefore I am". However, this does open up some questions. Here are a few that I have:

• What are thoughts exactly? Are they made up of matter? If not, do they really exist? Is existence determined by matter? Or is existence merely attributed to the idea that it exists? In other words, do we exist merely because we say so?
• You can only read your own thoughts. Therefore, you can only confirm your own existence, assuming that existence is indeed the capability to think. I for example don't know if other people truly have their own thoughts, or if other beings are merely constructs of my mind, since my mind determines what I feel, see, hear, etc. I could really be interacting with a sort of fake world of my own creation. Now, is our brain capable of creating such complex worlds and constructs in the first place, though?
• Once we are dead, what happens then? Assuming there is no afterlife, that means we cease to exist presumably. But what of our corpses? That leads back to the question of matter. Since our corpses would still exist at least in the short-term, is the decision is matter than we would still exist, but if it's of thoughts and minds that means we wouldn't.

Of course, the origins of the universe and world play in an important part in this too, it cannot be overlooked. However, we do not know the origins, so we can only assume. If we exist because we are here, then what if we're not? Is it the forest in the woods-kind of state? 

Then again, do we even want to know the truth? Let's assume that say I for example really am just interacting with figments of my imagination, or even say that I don't really exist and am merely a projection of someone else, only existing because they decide that I should exist. Should I take the potential information into consideration? Or should I continue living life as I was before? Is morality absolute no matter the reality? Or is it reality that makes morality? If the former, that would mean that committing atrocious acts in video games would be wrong, but if the latter, we would just have to assume that what we're interacting with, and what see think is real, is truly real. Because if not, it's essentially like a lucid dream where we are somewhat limited by the bounds we place on our selves subconsciously, but our conscious awareness of it allows us to act differently than we would otherwise.

What are your guys' thoughts?