Dark matter and vacuum How different would space have been if there was no dark matter?
Can space be considered vacuum even when occupied with abundant dark matter?
Non-baryonic dark matter (ie dark matter made of some particle that doesn't interact with light) had an important role in forming the large scale structure of the universe. The early universe was dominated by light radiation, and this would have prevented the early clumps of matter from growing. But dark matter isn't affected by radiation, so it can form clumps, which then attracts the normal matter towards it. Without dark matter, galaxies wouldn't have been able to form. You can read about structure formation on Wikipedia.

The second question is just about definitions. Generally, when we talk about a "vacuum" we mean a space with few molecules of gas or other atoms. This is because other particles, such as photons or dark matter particles don't contribute to pressure. A vacuum is a place with a very low pressure, not necessarily a place with no particles at all. So you can talk about a vacuum without needing to make a fuss over dark matter. However the actual density of dark matter is still very low: about the same as one hydrogen atom for every 3cm³. Even in the best vacuums that we can create on Earth, and even in interplanetary space, regular matter dominates.