How can space expand? How can space expand when it is only a perception of the separation between at least 2 objects. Isn't saying "space expands" implying it has properties?
 A: We model spacetime as a manifold and a metric. Broadly, the manifold gives us the dimensionality and connectivity while the metric provides a method of specifying distances. The equations of General Relativity allow us to calculate the metric from the stress-energy tensor (or vice versa if you're Miguel Alcubierre).
The point that jinawee is making in his comments is that even in the absence of matter or energy the Einstein equation can still be solved to give a spacetime. These solutions are known as vacuum solutions. For example a common and potentially very important vacuum solution is the gravitational wave.
So we do not need matter/energy to be present to have a notion of distance. All we need is the metric. In the particular case of the FLRW solution the metric looks like:
$$ ds^2 = -dt^2 + a(t)(dx^2 + dy^2 + dz^2) $$
where $a(t)$ is the scale factor and increases with time. This metric tells us that the distance between two spacetime points increases with time regardless of whether or not there are chunks of matter at those two points.
A: The (nearby) "separation between objects" you are referring to is the space-time  metric. A metric in cosmology describes the expansion of space on large angular scales (low $\ell$ on the angular power spectrum of the universe). Without going into the mathematics, the expansion of space is driven by cosmic inflation, and is affected by things the amount and distribution of matter and energy in the universe.
The origin of inflationary expansion is a major question in cosmology. "Space expanding" means that the scale of space itself is changing. By mentioning "properties of space" you are getting at the cosmological parameters in the current cosmological model (called $\Lambda CDM$) (e.g. $H_0$ the current rate of expansion, and $\Omega_k$ the curvature density) from the CMB power spectrum. So yes, space is described by physical parameters (some of which, like $H_0$ can change over time) that we measure on cosmological scales. 
BTW, if by "perception" you are referring to different observers (frames of reference), you should note that there are both observer-dependent and observer-independent notions of curvature (not all observers agree on the curvature of space due to the presence of mass/energy density). 
