Can you send an unsolicited resume to a company or do you have to wait until they are actively advertising?

Let's say a person graduated from college with a double major in journalism and a major in History. In the process of finishing grad school for public history. The person is looking for a developmental/ marketing position within a museum and also a position as a copy editor/journalist at a newspaper/ magazine. Of course, they send resumes to the ones that are actually advertising. What if you have a museum or newspaper/ magazine publisher that does not seem to actively advertise? Should you wait to apply for a position until one actually opens up? Or can you send an unsolicited resume and saying you are looking for future or upcoming job openings. How exactly/ What would be the proper way to convey that in a cover letter? What other facts or tips should you mention ? Even if some of the companies do advertising on the website most of the job postings may be out of date or old and then you send a resume and they do not call for an interview.