Open and Dense Substack I am looking for a definition of open and dense substack of a Deligne-Mumford stack $\mathcal X$. I have encountered this notion many times, but I am not able to find any references in which dense substacks are defined. 
The only thing I have found is the one on nlab - dense subtopos, but, as I don't know much about topos theory, I am wondering whether there is a definition which does not use the characterisation of DM-stacks as ringed topoi locally equivalent to the étale spectrum of a commutative ring.
Furthermore, is this notion stable under 2-pullback in the 2-category of stacks?
 A: Given an algebraic stack $\mathcal{X}$ there is a canonically associated topological space $|\mathcal{X}|$ of points of $\mathcal{X}$. A point is an equivalence class of morphisms $\text{Spec}(k) \to \mathcal{X}$, where $k$ is a field. Two morphisms $\text{Spec}(k) \to \mathcal{X}$ and $\text{Spec}(k') \to \mathcal{X}$ are equivalent if $\text{Spec}(k) \times_\mathcal{X} \text{Spec}(k')$ is nonempty (as an algebraic stack, i.e., there is a point). Given a morphism $f : \mathcal{X} \to \mathcal{Y}$ of algebraic stacks, the obvious map $|\mathcal{X}| \to |\mathcal{Y}|$ is continuous and if $f$ is smooth then it is open. This together with the requirement that $|\mathcal{X}|$ should be the usual topological space when $\mathcal{X}$ is a scheme, uniquely determines the topology.
Definition: An open substack $\mathcal{U} \subset \mathcal{X}$ is dense if $|\mathcal{U}| \subset |\mathcal{X}|$ is dense.
IMHO opinion this is the correct definition. More generally, topological properties should be, as much as possible, defined in terms of $|\mathcal{X}|$. For example a morphism $f : \mathcal{X} \to \mathcal{Y}$ of algebraic stacks is universally closed if for all morphisms $\mathcal{Z} \to \mathcal{Y}$ the map $|\mathcal{Z} \times_\mathcal{Y} \mathcal{X}| \to |\mathcal{Z}|$ is closed.
