Analysis:

In the four-dimensional space (not including time, it is a spatial dimension), the space independent of our three-dimensional space is like standing on the first floor of the three-dimensional space and another person standing on the second floor. In the four-dimensional space, you are in this three-dimensional space and he is in another three-dimensional space. Other three-dimensional spaces besides our three-dimensional space are called different dimensions.

This is also a theory of space. Generally speaking, we think our world is a three-dimensional space, and Einstein plus time has a four-dimensional space-time.

The theory of different-dimensional space is that there are some places that are not in our 4-dimensional space-time, but coexist with our 4-dimensional space-time, which cannot be found in our space-time. This place is another dimension.

This idea extends the four-dimensional space-time of the universe to five or more dimensions. String theory extends to more than ten dimensions, so I'm not familiar with it, so I didn't move it out.

All kinds of theories are imaginable, but we haven't proved them yet!

The dimension is opposite!

For example, our universe is a one-dimensional straight line in mathematics, which can extend infinitely to both ends, which can be said to be infinite.

However, when we jump out of a dimension and look at a two-dimensional plane, we will find that there are countless parallel straight lines next to our original infinite extension, which coexist with our straight lines. This can be understood as a parallel universe.

Similarly, you can add dimensions and extend them to three-dimensional space. ....

To sum up, by expanding dimensions, we can realize the phenomenon that some places are not in our four-dimensional space-time but coexist with our four-dimensional space-time, just as we live in a two-dimensional space and never feel the existence of a three-dimensional space.