Assume that this universe isn't eternal and there is no higher realm this universe is part of.
(It has a beginning and an end and is all that exists.)
Then it might only be up to us, observers, that the universe exists right now. Why?

Most of us believe that everthing in this world follows logical steps,
from the atomic scale to the astronomical scale, from physics to chemistry to biology
and so on. With this deterministic view we can imagine the Big Bang as step s0,
the next step would be s1, s2, s3, (which contains the formation of particles and galaxies)
and so on, until we reach step sN, the creation of a conscious observer.
From now on, it could still be a long chain of steps sN+1, sN+2, etc.
until the final endpoint sE is reached, the end of the universe.

Now if the universe isn't eternal, there is no universe at all before s0 or after sE, only in between. It's like a huge calculation being made and after the calculation is finished,
it has no use anymore, (because there is no higher realm which could make use of it, see above). It's like baking a cake with a recipe, step after step, and when the cake is ready, it has noone to eat it after. Or imagine a rendering process. From the very start of a 3D model a computer calculates the materials and the light to render a final image. But the image is never used after its creation.

Now imagine, that the universe had never created a conscious observer or imagine,
all of humanity and other living things ceased to exist. What is the universe now other than a predetermined calculation from s0 to sE, where calculation time doesn't play a role anymore because there is no observer which would check the steps in between (sN, sN+1,...)? So, in other words, sE is already reached in no time, and therefore s0 to sE is nothing more than a timeless (call it:) very huge algorithm, without a use because there is no higher realm to use the algorithm.

So without a conscious observer there are no steps sN, sN+1, .... to be observed in time. We could still argue that the universe would exist without humanity, it would do what it does, but if the determism is true, sE is already reached with the start of s0, so that it becomes obsolete, without any further use and without any observer to check the steps in between.

Without any conscious observer the end of the universe would be reached in no time,
and this equals no universe at all.
So without any conscious observer there is no universe at all.
It's only up to us, that the universe exists right now.