I'm not that familiar with WCF-RIA, but the documentation states that by default DomainServices do not have a physical .SVC file generated for them at Design Time, but rather have a virtual .SVC, which represents the WCF Service associated with the domain. For a given DomainService the path to its .SVC can be determined using the following convention:
[SilverlightApplicationBaseURI] + [DomainServiceFullName].svc
At RunTime when the first request (within a particular Application Domain) is made for a DomainService's .SVC file , the registered httpModules intercept the call and RIA Services writes out an in memory .SVC file on the fly.
The following MSDN Blogs provide good information:
http://blogs.msdn.com/b/saurabh/archive/2009/11/23/understanding-the-wcf-in-wcf-ria-services.aspx
http://blogs.msdn.com/b/brada/archive/2009/11/22/ria-services-a-domainservice-is-a-wcf-service-add-service-reference.aspx