Not quite. In ye olde days there wasn't any experimental evidence to show the rate of expansion was slowing, it was merely assumed by what we knew of gravity at the time. The thought was that because gravity is attractive and even though the strength of gravity declines greatly over distance, it is still a nonzero amount, it will eventually serve to bring any two objects together not being acted upon by an outside force. That outside forces are minimal in the larger universe, especially over large spans of time, it was assumed this attraction would serve to eventually pull the whole universe back together. Once we had the capacity to actually carry out experiments, we determined much to our surprise that, as you said, not only is the universe not slowing, it is expanding apart which implies a negative pressure on space, which is called dark energy.
Dark matter is something else, something meant to address a different problem. The universe is much, much heavier than the stuff we can see. For example, with galaxies spinning at their observed rate they should be flying apart; too much angular momentum in their arms. They aren't, and what that tells us is there is a much more intense force of gravity than just the visible matter in the galaxy to increase the mass and thus increase the gravitational attraction.
What's more, we have actually seen dark matter, kind of. After galaxy collisions, all sorts of galactic crap gets thrown everywhere; dust, stars, planets, and dark matter. When observing the remains of a galactic collision, we have discovered at least one region of space that produces a gravitational lense effect despite being observably empty. Gravitational lensing, for those not familiar with the term, is where gravity in a region bends light, causing light traveling past it to curve inward towards the gravity well. Like the lense on a camera, it will create a focal point, where, over great distances, light will converge which allows us to see things behind other things. What this all means is that there is an empty region of space with the mass of a galaxy; one of the destroyed galaxy's dark matter halos.
Now, don't get me wrong, we still have a lot we don't understand about them and we are learning more all the time. That said, a little perspective is called for; we aren't marking our maps of the galaxy "here be dragons" either.