Looking on the internet deeply has found these results:

We have not observed it. is the most popular phrase on the web.

We did not observ it. 10 results on the web	We have not observed it. 1852 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
It doesn't look like this phrase is very popular! The search could not find any examples on the internet.	2 days ago ... I see what he wanted to assert. But why do we need the cat to be dead and alive? As long as we have not observed it, it could be dead OR alive ... Oct 29, 2015 ... "There is no need for undue alarm because we are on a course to recovery, but we have not observed it yet," said WMO scientist Geir Braathen, ... possibilities for the existence of life or why we have not observed it as of yet, since none of these approaches have been successful. It may now be possible to ... ... some other form of existential meaning, does not exist after death because we have not observed it (we discuss this limitation in some detail in our other book).

We did not observ it.

10 results on the web

We have not observed it.

1852 results on the web

More popular!

It doesn't look like this phrase is very popular! The search could not find any examples on the internet.

2 days ago ... I see what he wanted to assert. But why do we need the cat to be dead and alive? As long as we have not observed it, it could be dead OR alive ...
Oct 29, 2015 ... "There is no need for undue alarm because we are on a course to recovery, but we have not observed it yet," said WMO scientist Geir Braathen, ...
possibilities for the existence of life or why we have not observed it as of yet, since none of these approaches have been successful. It may now be possible to ...
... some other form of existential meaning, does not exist after death because we have not observed it (we discuss this limitation in some detail in our other book).