Most reputable magazines, as with TV or movie production companies and book publishers, do not accept unsolicited manuscripts.
Once you send an inquiry outlining your concept and an estimated word-count for the final product, you sit back and wait to see whether or not your target is interested. That inquiry has to be accompanied by a Self Adressed Stamped Envelope. (I used bolding there because it is known industry-wide as an SASE.)
A preliminary approach is required because they absolutely will not open anything unsolicited for the very reason that you are being secretive about. There are far too many people who think that they're the second coming of Leonardo Da Vinci. If they submit something of their own volition, they are more than eager to scream "plagarism" if that company publishes or produces something that shares a couple of words or even a vague concept that the would-be author submitted. "Nuisance" lawsuits were prevalent until the current policy became the norm. Anything that you send in on your own is automatically returned, unopened, to prove later that the company couldn't have stolen your idea since they never knew about it.
My strongest recommendation is that you either buy or peruse at your local library a hefty tome entitled "Writers' Market" which is published by Writers' Digest Books. The library is a better approach, since the book is quite expensive. (The last time that I bought one was in '78, and it cost about $45 even back then.)
There are thousands of entries which provide contact information, overall market health, etc..
By the bye, unless your contract stipulates otherwise, mags such as PopSci purchase only "first North American serial rights" on a routine basis. That gives them the right to publish (once only) your article in the US, Canada and (I think) Mexico. That means that any subsequent re-printing has to be negotiated with you again.
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