The speed at which the Universe is expanding is not a constant.
This is a very interesting consequence of the presence of dark energy in our Universe, and gives us an interesting view into the very distant future of our Universe.
Dark energy is a descriptive term we have applied to a force which is responsible for the observed expansion of the Universe. We can see that the Universe is expanding, by measuring the apparent speeds of objects in the Universe, all of which appear to be receding at a rapid clip. Considering that we’re not at a special place in the Universe, this observation is best explained by every object drifting away from every other object. Given that gravity is also present in our Universe, some other force must be acting upon each and every object in our Universe in order to counteract gravity and keep them on their paths to an increasingly isolated Universe.
This force which counteracts gravity has been dubbed Dark Energy, and exactly what it is and how it operates is still extremely poorly understood. However, based on our observations, it must make up about 68% of all the energy present in the Universe to be able to do what we observe it doing – pushing all galaxies which aren’t tied to each other by gravity further apart from each other.