Dark Matter

I found something really interesting while turning the pages of my old note book of astronomy club. It was an article about dark matter which I wrote during the winter vacation of 2013. It was the very first year when I heard of the term dark matter and dark energy. When I asked my teacher about it she said the dark matter is like a glue to our universe. The matter we know i.e stars, planets, galaxies, atoms and us accounts only 5% of the known universe.  25% of our universe is dark matter and the rest 70% is dark energy.

What is dark matter? Can we see it? if not then why? Why do scientists think that it exists?