kernel_task uses up over 50% cpu usage until I open Activity Monitor My mac is slower than it used to be a week ago. I may have downloaded some sort of malware, but I did a Malwarebytes scan and it found absolutely nothing on my computer. I also personally scanned through all the files and found nothing suspicious.

The problem is that when I open Activity Monitor, it displays a very high cpu usage for a second, then it immediately goes down. I have seen examples of this where the virus only activates when the cpu usage is not shown.

I am completely up to date with my OS and software updates, so I am very certain that the operating system itself is not the problem.
Maybe you could get a CPU monitor to check the status beside the Activity Monitor. This way you could tell if its really tied to activity monitor. I have an app called  CPU LED which shows a string of colors indicating CPU usage but there are several in addition to this. Doing an app store search on CPU Monitor brings up about 50 hits. You can scan through these to find one that suits your needs. Then if it is tied to Activity Monitor you could download a mac virus checker with a free trial period and run it on your machine.