Why doesn't the cosmological constant cause black holes to form? The cosmological constant acts as matter with positive energy but negative pressure. Then a region $R$ should have energy $E\sim \Lambda r^3$. On the other hand, we know that the energy for a black hole is $E_{BH}\sim r$, then even $\Lambda$ is very small, for large enough region $R$, the energy will be large enough to become a back hole. Specifically, the Schwarzchild radius is $r_{s}=2E\sim 2\Lambda r^3$, when $r_{s}> r$, namely $2\Lambda r^2\gtrsim 1$ then a black hole should be formed.
What's wrong with the analysis here? 
 A: It's a common misconception that the source of gravity is just mass. In general relativity the spacetime curvature is related to an object called the stress-energy tensor. This is a second rank tensor, and in most cases we write it as a 4 x 4 matrix that looks like this (image from Wikipedia):

The number highlighted in red, $T^{00}$, is what we normally think of as mass, and in many cases that's the only number that matters. However in the case of dark energy we have also to consider the three diagonal elements $T^{11}$, $T^{22}$ and $T^{33}$ which behave like a pressure. Specifically for dark energy they behave as a negative pressure, and the effect of this is to drive an accelerated expansion.
What happens to the universe depends on the balance between the $T^{00}$ term and the three pressure terms. If $T^{00}$ is very large and the pressure terms are negligable we do get a collapse, though in the case of the universe we get a Big Crunch not a black hole. If the pressure terms are negative and their magnitude is very large then we get cosmological inflation. Right now $T^{00}$ and the pressure terms are comparable, so the expansion of the universe is accelerating fairly gently.
A: I can offer you a simple explanation: 
If you look at the Friedmann equation $$\frac{H^2}{H_0^2} = \Omega_R a^{-4} + \Omega_M a^{-3} + \Omega_k a^{-2} + \Omega_{\Lambda},$$ you will see that the cosmological constant DOES NOT act like matter because it does not scale as matter does. What this means is that the density of dark energy is uniform and does not depend on the scale factor of the universe meaning its density does not increase like normal matter when you compress it. It just remains constant. If you compress the volume, you just end up reducing the amount of vacuum energy contribution in that shrinking volume. So there is no well defined meaning of Schwarzschild radius for the cosmological constant. This as you can see is not how black hole formation works. 
