Why Does Dark Energy Make the Universe Accelerate? by Sean Carroll has a good explanation.
The usual (wrong) way to explain this is to point out that dark energy has “negative pressure.” ...
The right way is to not mention pressure at all, positive or negative. ... Dark energy is persistent, which imparts a constant impulse to the expansion of the universe, which makes galaxies accelerate away. No negative pressures, no double-talk.
The problem with just stating "negative pressure" is the immediate conclusion is that here is no acceleration! Any equal pressure from all around an object such as a galaxy has no net force. So it has to be gravity from the negative pressure and that needs justification.