What does it mean if a domain does not have any WHOIS information available?

For example, I found two websites that mimic my domain name perfectly eg mydomainname.com, with the two offending websites being mydomainname.bw and mydomainname.br. However the domains are not linked to any IP, nor do they have any WHOIS information available. The only way I can see that they are even registered is if I go to a domain seller that deals with .bw and .br TLDs, they tell me that the domain cannot be purchased. Does that mean the domain is just "reserved" somehow and nobody registered it?