Thanks for the replies! Yes, X is the nxk matrix of explanatory variables such that y=Xβ+ε. I think I understand it now. Variables in X do not necessarily follow a stochastic process, and even if they do, since all variability of y is explained by ε in the model, the independent variables...
So in computing the variance-covariance matrix for β-hat in an OLS model, we arrive at
VarCov(β-hat)=(σ_ε)^2E{[X'X]^-1}
However, I'm incredulous as to how X is considered non-stochastic and how we can just eliminate the expectation sign and have
VarCov(β-hat)=(σ_ε)^2[X'X]^-1
I'm...