also I'd say that in modern language "тому назад" is used far less often than "назад". By no means archaic, the use of "тому назад" has fallen tenfold over the last 100 years. You should know that they are interchangeable in the meaning of "(some time) AGO", and you should understand "тому назад" when you see it or hear it. In practice, however, you would use "назад" yourself, and so would do native speakers most of the time. The catch is, if you are reading books, you aren't ALWAYS exposed to late 20th-early 21th century language.